Báo cáo khoa học: Binding and activation of nitric oxide synthase isozymes by calmodulin EF hand pairs

Binding and activation of nitric oxide synthase isozymes by calmodulin EF hand pairs Donald E. Spratt1, Elena Newman1, Jennifer Mosher1, Dipak K. Ghosh2, John C. Salerno3 and J. G. Guillemette1

1 Department of Chemistry, University of Waterloo, ON, Canada 2 Department of Medicine, Duke University and VA Medical Center, Durham, NC, USA 3 Biology Department, Rensselaer Polytechnic Institute, Troy, NY, USA

Keywords activation; binding; calmodulin; nitric oxide; nitric oxide synthase

Correspondence J. G. Guillemette, Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Fax: +1 519 746 0435 Tel: +1 519 888 4567 ext. 5954 E-mail: jguillem@sciborg.uwaterloo.ca

(Received 23 December 2005, revised 10 February 2006, accepted 20 February 2006)

doi:10.1111/j.1742-4658.2006.05193.x

Calmodulin (CaM) is a cytosolic Ca2+ signal-transducing protein that binds and activates many different cellular enzymes with physiological rele- vance, including the nitric oxide synthase (NOS) isozymes. CaM consists of two globular domains joined by a central linker; each domain contains an EF hand pair. Four different mutant CaM proteins were used to investi- gate the role of the two CaM EF hand pairs in the binding and activation the mammalian inducible NOS (iNOS) and the constitutive NOS of (cNOS) enzymes, endothelial NOS (eNOS) and neuronal NOS (nNOS). The role of the CaM EF hand pairs in different aspects of NOS enzymatic function was monitored using three assays that monitor electron transfer within a NOS homodimer. Gel filtration studies were used to determine the effect of Ca2+ on the dimerization of iNOS when coexpressed with CaM and the mutant CaM proteins. Gel mobility shift assays were performed to determine binding stoichiometries of CaM proteins to synthetic NOS CaM-binding domain peptides. Our results show that the N-terminal EF hand pair of CaM contains important binding and activating elements for iNOS, whereas the N-terminal EF hand pair in conjunction with the cen- tral linker region is required for cNOS enzyme binding and activation. The iNOS enzyme must be coexpressed with wild-type CaM in vitro because of its propensity to aggregate when residues of the highly hydrophobic CaM- binding domain are exposed to an aqueous environment. A possible role for iNOS aggregation in vivo is also discussed.

there is significant interest

activation of over 50 intracellular proteins [2]. Because of the manifold and diverse roles of CaM in intracellu- in better lar signaling, understanding the structural basis of its recognition of target proteins.

Abbreviations CaM, calmodulin; nCaM, CaM residues 1–75; cCaM, CaM residues 76–148; CaMNN, engineered protein in which CaM residues 82–148 have been replaced by the sequence of CaM residues 9–75; CaMCC, engineered protein in which CaM residues 9–75 have been replaced by the sequence of CaM residues 82–148; central helix linker, CaM residues 76–81; CaM-TnC, CaM-troponin C chimera; NOS, nitric oxide synthase; •NO, nitric oxide; cNOS, constitutive NOS enzymes; eNOS, endothelial NOS (NOSIII); iNOS, inducible NOS (NOSII); nNOS, neuronal NOS (NOSI).

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Calcium (Ca2+) is an important signaling molecule involved in diverse physiological processes such as motility, neurotransmission, memory, fertilization, cell proliferation, cell defense, and cell death [1]. Calmodu- lin (CaM), a ubiquitous 17-kDa cytosolic protein, is a major cellular Ca2+ sensor which rapidly regulates co-ordinated intracellular processes through the CaM is a 148-amino-acid protein consisting of two linker region. globular domains joined by a central

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Calmodulin domain activation of NOS

activation [13]. The primary requirements for iNOS activation were associated with EF hands 2 and 3. We now report on the CaM-dependent activation of mammalian NOS isozymes focusing on iNOS–CaM interactions.

Results

Protein expression and purification

spectrometer

The mutant CaM constructs described in Experimental procedures produced good independent expression ran- ging from 8 to 26 mg protein per liter of medium, depending on the CaM mutant. Purified CaM con- structs appeared over 95% homogeneous on SDS ⁄ PAGE (15% gel) (Fig. 1). Electrospray ionization MS con- time-of-flight on a quadrupole firmed homogeneity and ruled out post-translational modification.

Each domain of CaM contains an EF hand pair. The C-terminal EF hand pair has an affinity for Ca2+ (Kd ¼ 10)6 m) 10-fold greater than the N-terminal EF hand pair (Kd ¼ 10)5 m) [3]. Previous studies involving exchange of EF hand pairs within CaM have been per- formed to study specific interactions of CaM domains with target enzymes during binding and activation [4,5]. The present investigation, designed to further assess the role of the two CaM EF hand pairs in the binding and activation of nitric oxide synthase (NOS; EC 1.14.13.39), used four different mutant calmodu- lins: nCaM, cCaM, CaMNN, and CaMCC. The trun- cated nCaM construct includes only the N-terminal EF hand pair without the central linker region (resi- dues 1–75), and the complementary cCaM construct includes only the C-terminal EF hand pair including the central linker region (residues 76–148). In addition, CaMNN contains two repeats of the N-terminal EF hand pair (residues 1–81, 9–75), and CaMCC contains two repeats of the C-terminal EF hand pair (residues 1–8, 82–148, 76–148). CaMNN and CaMCC EF hand pairs are both connected via the central linker region (residues 76–81).

kDa

1

2

3

4

5

6

45

30

The iNOS enzyme was coexpressed with CaM or a mutant CaM construct. Coexpression with wild-type CaM produced the highest yields of purified iNOS (3.2 mgÆL)1). Coexpression of iNOS with CaMNN yields 2 mgÆL)1 whereas coexpression with nCaM gave 0.6 mgÆL)1. Expression of iNOS with cCaM and CaMCC gave the lowest yields of 0.2 mgÆL)1. CaM constructs containing the N-terminal EF hand pair produced higher yields of iNOS, indicating better pro- tection of the CaM binding region than provided by iNOS the C-terminal EF pair. Visible spectra of

20.1

14.4

Fig. 1. SDS ⁄ PAGE (15% gel) of the purified mutant CaM proteins. A 5-lg sample of each purified CaM protein was loaded in a stand- ard SDS ⁄ PAGE buffer containing 5 mM EDTA. Lane 1, low molec- ular mass protein standard (Bio-Rad); lane 2, wild-type CaM; lane 3, nCaM; lane 4, cCaM; lane 5, CaMNN; lane 6, CaMCC.

catalytic heme The NOS enzymes produce nitric oxide (•NO), which participates in a wide variety of processes such as neurotransmission, vasodilation, and immune defense [6]. The three mammalian isoforms are homo- dimeric; each monomer consists of a multidomain C-terminal reductase region and an N-terminal oxyg- enase domain. The reductase domains bind NADPH, FAD, and FMN, and the oxygenase domain con- tains binding sites tetrahydrobiopterin for heme, (H4B), and the substrates l-arginine and molecular the oxygen [7]. A CaM-binding domain separates raised Ca2+ oxygenase and reductase regions. At concentrations, CaM binds to constitutive NOS (cNOS) enzymes, neuronal NOS (nNOS) and endo- thelial NOS (eNOS), enabling conformational chan- ges in the reductase domains that facilitate electron transfer from NADPH through reductase-associated flavins in the oxygenase to the domain [8–11]. The inducible NOS (iNOS)

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isozyme is transcrip- tionally regulated in vivo by cytokines. CaM–iNOS interactions are not well studied because iNOS could originally only be purified when coexpressed with wild-type CaM [12]. We overcame this problem by coexpressing iNOS with mutant CaM proteins and successfully produced active enzyme [13]. Our previ- ous study, using CaM-troponin C chimera (CaM- TnC) as a probe of specific NOS–CaM interactions, demonstrated that the requirements for iNOS activa- than those for cNOS tion were far less stringent

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coexpressed with CaM constructs (not shown) were indistinguishable from those of iNOS coexpressed with wild-type CaM, indicating proportionate heme and fla- vin content. All of the CaM constructs showed pro- duction of active iNOS by the oxyhemoglobin capture assay; however, the activity of iNOS coexpressed with cCaM was very low.

cNOS activation by CaM proteins

The nNOS and eNOS enzymes displayed similar but not fully equivalent activation profiles when associated with different CaM mutants. The only CaM construct able to activate •NO production by nNOS was CaM- NN; nCaM, cCaM and CaMCC produced little or no activity (Table 1). These results correlate well with pre- vious reports [4,14]. None of the CaM constructs acti- vated •NO production by nNOS in the presence of 250 lm EDTA, consistent with our study of NOS acti- vation by CaM-TnC chimeras [13].

of exogenous unbound flavins added to the reaction buffer of the assay [15]. The rate of NADPH oxidation by nNOS activated by wild-type CaM or mutant CaM proteins shows the same degree of enhancement as •NO production. This indicates that any redox cycling requires the reduction of free flavins by the NOS reductase domain. The ratio of NADPH oxidation to •NO synthesis was the same for nNOS activated by CaM and CaMNN. NADPH oxidation by eNOS acti- vated by either CaM or the mutant CaM proteins did not show the same order as observed for the produc- tion of •NO (Table 1). The eNOS enzyme showed greater electron uncoupling than nNOS for wild-type CaM and CaMNN. This suggests that eNOS may be more susceptible than nNOS to the uncoupling of elec- trons from NADPH oxidation to •NO production when activated by mutant CaM proteins, similar to our findings in a previous study [13]. Only CaM con- structs containing the N-terminal domain of CaM, nCaM and CaMNN, activated cytochrome c reduction by nNOS. Constructs lacking the N-terminal domain of CaM, cCaM and CaMCC produced little or no activation of electron transfer to cytochrome c. Specific residues in the N-terminal domain of CaM appear to be required for activation of electron transfer in nNOS.

CaMNN was also the only CaM construct that pro- duced appreciable •NO production from eNOS. CaM- CC activated eNOS to a much smaller extent (20%), while nCaM and cCaM produced little or no activity (Table 1). Although the order of activation for eNOS was similar to that of the nNOS enzyme, CaMNN fully activated eNOS but only activated nNOS to (cid:1) 80% when compared to wild-type CaM. None of the CaM proteins activated •NO synthesis by eNOS in the presence of EDTA.

Table 1. CaM protein-dependent activation of cNOS enzymes. The oxyhemoglobin capture assay used to measure the rate of CaM-activated •NO production, the cytochrome c assay and the NADPH oxidation assay were performed in the presence of either 2 lM wild-type or mutant CaM protein and either 200 lM CaCl2 or 250 lM EDTA, as indicated. The activities obtained with the respective enzyme bound to wild-type CaM at 25 (cid:1)C in the presence of 200 lm CaCl2 were all set to 100%. The activities for nNOS bound to CaM were 45.5 min)1 (•NO synthe- sis), 142 min)1 (NADPH oxidation) and 917.5 min)1 (cytochrome c reduction). The activities for eNOS bound to CaM were 11 min)1 (•NO synthesis), 30 min)1 (NADPH oxidation) and 50.7 min)1 (cytochrome c reduction). NAA, No apparent activity.

Neuronal NOS

Endothelial NOS

NADPH oxidation (%)

Cyt c reduction (%)

CaM protein

•NO production (%)

NADPH oxidation (%)

Cyt c reduction (%)

•NO production (%)

100 ± 6 37 ± 3

100 ± 4 5 ± 3

100 ± 2 17 ± 1

100 ± 2 5 ± 1

NAA

100 ± 5 NAA NAA

NAA

90 ± 5

81 ± 3

NAA 115 ± 4 4 ± 3

NAA 111 ± 3 43 ± 1

98 ± 4 17 ± 3

100 ± 2 6 ± 2 4 ± 3 93 ± 4 5 ± 2 6 ± 3

NAA NAA

NAA NAA

NAA

NAA

CaM nCaM cCaM CaMNN CaMCC CaM (EDTA)

NAA

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The activation of nNOS and eNOS by wild-type CaM resulted in an NADPH consumption to •NO production ratio of more than 3 instead of the theoret- ical ratio of 1.5. High NADPH consumption rates with eNOS were previously attributed to redox cycling The activation of electron transfer within the reduc- tase domains of eNOS showed a similar trend to the results obtained for nNOS cytochrome c reduction. The details of activation by CaM constructs are not identical; with eNOS, CaMNN is a slightly more potent activator of cytochrome c reduction than wild- type CaM, whereas nCaM produces markedly lower rates of cytochrome c reduction. Notably, CaMCC promoted electron transfer in the reductase domains of

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in the reductase domains of pressed with only nCaM, the central linker region of CaM may play an important role in the binding and activation of iNOS. Significant eNOS but not nNOS. It appears that specific residues in the N-terminal domain are important for electron transfer the cNOS enzymes, and that the central linker region may also play a pivotal role.

iNOS activation by CaM proteins

levels of •NO synthesis were restored when excess wild-type CaM was added to iNOS coex- pressed with any of the four mutant CaM proteins in the presence of EDTA (Table 2). These results indicate that a Ca2+-dependent reorganization of the bound mutant can allow binding and activation by the addi- tion of excess native CaM.

The coexpression of iNOS with CaM constructs con- taining the N-terminal domain of CaM, nCaM and CaMNN resulted in reproducible •NO production rates of (cid:1) 70% in the presence of Ca2+ (Table 2). In contrast, CaM proteins consisting of only the C-ter- minal domains of CaM resulted in reproducible •NO production rates of less than 50%. The addition of excess wild-type CaM to iNOS coexpressed with each of the CaM constructs did not result in any significant change in the activity of the enzyme, indicating that the CaM-binding sites were saturated with mutant CaM proteins and do not exchange rapidly with CaM in solution (results not shown).

Comparing the stimulation of NADPH oxidation by the CaM constructs showed a pattern comparable to the activation of •NO synthesis by iNOS (Table 2). Coexpression of CaM, nCaM, CaMNN and CaMCC with iNOS resulted in a stoichiometry of about 1.5 NADPH molecules oxidized per •NO molecule formed in the presence of Ca2+, whereas cCaM showed a higher ratio, probably due to electron uncoupling from •NO production. In the presence of a large excess of EDTA, only iNOS coexpressed with wild-type CaM maintained tightly coupled electron transfer, whereas iNOS coexpressed with the any of the CaM constructs oxidized more NADPH per •NO produced. These results indicate that •NO production by iNOS coex- pressed with CaM and mutant CaM proteins is more tightly coupled than •NO production by cNOS enzymes. This tendency is especially marked in the presence of Ca2+, but is also evident when Ca2+ has been removed with EDTA.

important

Table 2. CaM protein activation of iNOS. •NO synthesis, cytochrome c reduction and NADPH oxidation rates were measured as described in Table 1 except that no exogenous CaM was added to the assay. Each assay was performed in the presence of either 200 lM CaCl2 or 250 lM EDTA as indicated. The activities obtained for iNOS coexpressed with CaM and assayed in the presence of 200 lM CaCl2 at 25 (cid:1)C were all set to 100% and were 47 min)1 (•NO synthesis), 101 min)1 (NADPH oxidation) and 1397 min)1 (cytochrome c reduction). NAA, No apparent activity.

•No Production

NADPH oxidation

Cyt c reduction

(%)

CaM protein

250 lM EDTA (%)

(%)

250 lM EDTA (%)

(%)

250 lM EDTA (%)

500 lM EDTA with 2 lM CaM (%)

66 ± 2

NAA NAA

96 ± 6 28 ± 3 20 ± 3 58 ± 3 38 ± 3

100 ± 2 109 ± 3 62 ± 1 133 ± 4 69 ± 2

94 ± 1 115 ± 2 77 ± 2 133 ± 1 82 ± 3

100 ± 2 71 ± 2 12 ± 1 74 ± 7 54 ± 1

100 ± 4 80 ± 7 44 ± 3 48 ± 3 75 ± 1

23 ± 1 7 ± 1

81 ± 3 59 ± 2 31 ± 2 94 ± 1 80 ± 2

CaM nCaM cCaM CaMNN CaMCC

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With the use of the cytochrome c assay to monitor electron transfer from the flavins to an exogenous elec- tron acceptor, the iNOS enzymes coexpressed with the CaM proteins, nCaM and CaMNN, reproducibly dis- played over 100% of the maximal activity obtained with wild-type CaM in the presence of Ca2+ and EDTA. The addition of 250 lm EDTA to chelate Ca2+ resulted in a significant decrease in stimulation of •NO production by all of the CaM constructs, with the most noteworthy being iNOS coexpressed with nCaM, which decreased from 70% normal •NO pro- duction to no apparent activity (Table 2). iNOS coex- pressed with CaMNN experienced a similar trend, but still retained 25% activity in the presence of EDTA. Little or no activity was observed for iNOS coexpressed with cCaM and CaMCC in the presence of EDTA. These results show that the N-terminal EF hand pair of CaM contains elements required for the activation of iNOS. As iNOS coex- pressed with CaMNN maintains some activity in the in contrast with iNOS coex- presence of EDTA,

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Enzyme quaternary structure

viously reported by many researchers, all of the elution profiles obtained for iNOS coexpressed with CaM con- tain a contaminating peak apparently representing a proteolytic cleavage fragment [16].

Gel filtration studies were performed to investigate the effects of metal ion chelation by EDTA on iNOS dimerization. Blue dextran (molecular mass 600 kDa) was used to show that the calibrated column had a void volume of 8.35 mL. The iNOS enzymes were incubated for 5 min in the presence of 5 mm EDTA before loading on the gel filtration column equilibrated with buffer containing 250 lm EDTA. The iNOS enzyme coexpressed with wild-type CaM was mainly in the form of a dimer and was not sensitive to Ca2+ depletion or the addition of excess CaM (Fig. 2A–C). The iNOS dimer was eluted at a volume of 11.22 mL, corresponding to a molecular mass of (cid:1) 290 kDa, whereas excess native CaM was eluted at 16.0 mL, which represents a protein of less than 30 kDa. As pre-

The iNOS enzyme coexpressed with nCaM was used in these experiments because it showed the greatest Ca2+ sensitivity. In the presence of Ca2+, the elution profile showed that the enzyme sample consisted of a mixture of monomers and dimers (Fig. 2D). The iNOS monomer was eluted at 12.45 mL, corresponding to (cid:1) 160 kDa. Chelation of Ca2+ resulted in the disap- pearance of the dimer, a substantially decreased enzyme peak, and a significant increase in aggregated protein that was eluted in the void volume (Fig. 2E). The increased aggregated protein consists of iNOS as the void volume shows a strong heme absorbance at iNOS coex- 398 nm. The apparent aggregation of pressed with nCaM in the presence of EDTA accounts for the lost enzyme activity (Table 2). Figure 2F shows the elution profile for iNOS coexpressed with nCaM treated with EDTA in the presence of excess wild-type CaM. The addition of the excess native CaM appears to prevent the apparent aggregation of the protein. This is likely to occur because of a change in the inter- action of nCaM with the enzyme that may expose regions of the protein that are prone to aggregation. These results are fully consistent with the activation properties of the enzyme when excess CaM is added in the absence of Ca2+ (Table 2).

Non-denaturing gel electrophoresis of iNOS coex- pressed with either wild-type CaM or nCaM indicates that the aggregation of iNOS occurs when preincubat- ed with higher concentrations of EDTA. Consistent with the result observed with gel filtration, EDTA- induced aggregation is diminished when the enzyme is simultaneously incubated with excess wild-type CaM (results not shown).

Fig. 2. Gel filration elution profiles of iNOS coexpressed with CaM proteins. Absorbance at 280 and 398 nm are shown as solid and dashed lines, respectively. D, M, and CaM represent NOS dimer, monomer, and excess CaM, respectively. (A) 80 lg purified iNOS coexpressed with CaM was loaded on a Superdex 200 HR column equilibrated with 50 mM Tris ⁄ HCl, pH 7.5, containing 10% glycerol, 0.1 M NaCl, and 1 mM dithiothreitol (TGND buffer). (B) Profile of iNOS coexpressed with wild-type CaM incubated with 5 mM EDTA for 5 min before loading on the column equilibrated with TGND buf- fer in the presence of 250 lM EDTA, and (C) profile of iNOS co- expressed with wild-type CaM under the same conditions as in (B), with 10-fold excess wild-type CaM added to the 5 min incubation mixture. (D) 25 lg purified iNOS coexpressed with nCaM in the same conditions as in (A). (E) Profile of iNOS coexpressed with nCaM under the same conditions as in (B). (F) Profile of iNOS co- expressed with nCaM under the same conditions as in (C). Results shown are representative of three similar experiments.

similar the

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The structures of synthetic peptides of minimal length derived from the CaM-binding regions of the three mammalian NOS enzymes were studied by CD spectroscopy (results not shown). The iNOS and eNOS peptides alone in solution had predominantly random coil conformations. In the presence of Ca2+, the addi- tion of an equimolar ratio of CaM to either of the peptides resulted in a significant increase in a-helical content. The addition of equal amounts of either iNOS or eNOS peptide to nCaM also resulted in an increase in the a-helical content of both the iNOS and eNOS peptides. As expected, the addition of excess EDTA to eNOS resulted in mainly random coil structure. Nota- iNOS peptide conditions, bly, under retained some a-helical structure suggesting that the nCaM protein is able to bind the iNOS peptide in the

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presence of excess EDTA. Our results show for the first time that, whereas the N-terminal EF hand pair of CaM alone can accommodate the Ca2+-independ- ent binding of wild-type CaM for iNOS, apo-nCaM is not able to activate the enzyme.

constructs under these conditions (results not shown). The observation of Ca2+-dependent complex forma- tion by CaM incubated with iNOS CaM-binding pep- tides has been previously reported using this assay [17]. The shorter iNOS peptides used in these two studies do not bind strongly enough to show complex forma- tion using this assay; however, we did observe proof of binding based on CD analysis, consistent with previ- ously reported studies [17,18].

Discussion

target proteins

The structure of CaM interacting with target peptides derived from sources including myosin light chain kinase, CaM-dependent kinase, and eNOS has been shown to consist of two EF hand pairs linked by a short connector wrapped around a helical target. This model has provided a general mechanism for how CaM binds and activates [19–21]. Recent studies have shown that CaM is able to take on many different conformations when bound to diver- gent target proteins [22–25]. Gel mobility shift assays were performed to investi- gate the binding of the three NOS peptides to the dif- ferent CaM constructs. Complex formation between the peptide and CaM construct is monitored by the shift in the mobility of the CaM protein with increas- ing peptide concentration (Fig. 3). The mobility of the complexes formed reflects a change in conformation of the protein upon binding to the target peptide in addi- tion to a change in the molecular mass of the complex. Stoichiometric binding in a 1 : 1 ratio was observed for CaM with all three peptides in the presence of Ca2+. In contrast, nCaM, CaMNN and CaMCC showed strong binding to the iNOS peptide but in a 2 : 1 protein to peptide ratio. The 2 : 1 ratio observed for the three CaM constructs indicates that the iNOS peptide can accommodate more than one protein. In the cCaM protein appears to bind very contrast, weakly to the iNOS peptide.

The cCaM and CaMCC constructs seem to only weakly interact with the cNOS peptides resulting in a streaked protein migration (Fig. 3), whereas the nCaM protein does not interact at all. Only CaMNN shows binding using this assay, but it never goes to comple- tion. These results are consistent with activity assays shown above.

Fig. 3. Gel mobility shift assay with synthetic NOS peptides binding to CaM proteins. CaM, CaMNN, and CaMCC (20 lM) incubated with increasing molar ratios of peptide to CaM of 0, 0.25, 0.5, 0.75, 1, 1.5, 2, 4, 6, and 8 in the presence 0.2 mM CaCl2. The nCaM and cCaM mutants (60 lM) were incubated with molar ratios of 0, 0.125, 0.25, 0.375, 0.5, 0.75, 1, 2, 4, and 8 using the same conditions as described for CaM. The samples were then analyzed by PAGE (15% acrylamide) containing 0.375 M Tris ⁄ HCl, pH 8.8, 4 M urea, and 0.2 mM CaCl2 and visualized with Coomassie Blue R-250.

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Our previous kinetic study involving all three iso- forms of NOS with CaM-TnC chimeras demonstrated that the roles of the four EF hands in the binding and activation of the cNOS and iNOS enzymes are distinct [13]. Replacement of any CaM EF hand by its TnC cognate resulted in significantly decreased •NO synthe- sis by cNOS; in contrast with the iNOS results, EF hand 2 was the least sensitive even though it diverges furthest in TnC. These results could be interpreted in terms of the tethered shuttle model, in which the FMN binding domain is a mobile element connecting the oxygenase domain with the reductase complex [11]. In cNOS, cytochrome c reduction required only that the An investigation of complex formation was per- formed using the apo forms of the CaM constructs incubated with each of the NOS peptides by incuba- ting the samples in the presence of 1 mm EDTA. No mobility shifts were observed for any of the apo-CaM

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FMN binding domain be released from the reductase complex, but •NO production also required that CaM mediate the interaction of the FMN binding domain with the oxygenase domain.

in the classical closed configuration,

production by cNOS enzymes. In addition, it is poss- ible that the C-terminal EF hands of CaMNN occupy part of the binding domain usually filled by the C-ter- minal EF hand pair. The nNOS enzyme is incom- pletely activated when bound to CaMNN, but CaMNN fully activates •NO synthesis by eNOS and reproducibly activates eNOS NADPH oxidation and cytochrome c reduction more efficiently than wild-type CaM. CaMCC also slightly activates •NO production ((cid:1) 15%), indicating that eNOS is not as selective for specific elements in the N-terminal CaM domain as nNOS. Our findings correlate well with previous significant differences between cNOS reports of enzyme activity and electron transfer using mutant CaM proteins and the oxidation of CaM methionine residues [13,26,27].

linker region, importance. Several classes of suggests

If we initially assume that CaM binds to the NOS enzymes the N-terminal EF hands bind the C-terminus of the target peptide with EF hand 2 in closest contact with the reductase domains. The central helix is composed of the C-terminal and N-terminal helices of EF hand units 2 and 3, respectively, and is broken in the closed state. The C-terminal EF hands are associated with the N-terminus of the target peptide. They are located toward the oxygenase end of the CaM binding site, but the N-terminus of EF hand 3 is positioned to interact with reductase elements and forms contacts with the target. The spacer linking the CaM binding site to the oxygenase domain is not conserved and of variable length, suggesting that interactions between EF hand 4 and the adjacent oxygenase domain are of lesser interaction between CaM constructs and the target are possible starting from the classical model. nCaM might be assumed to bind to the recognition sites for N-terminal EF hands, but a minority population of alternative bound species could also exist. In the same way, cCaM would be expected to bind preferentially to recognition sites for the C-terminal EF hands, but a minority pop- ulation of alternative bound species may be present, including states in which cCaM is bound to the nCaM recognition site. Electron transfer through the reductase domains of nNOS to cytochrome c was stimulated by constructs containing the N-terminal EF hand pair of CaM, although CaMNN, with four EF hands and the CaM is much more effective than central interactions that nCaM (Table 1). This between the N-terminal domain of CaM and the reductase domains of nNOS promote the release of the FMN domain from its shielded position in the reduc- tase complex, allowing efficient electron transfer to an exogenous acceptor. However, in the case of nCaM, electron transfer is partially uncoupled from •NO pro- duction, suggesting that the presence of the central lin- ker region (and perhaps of any C-terminal EF hand) is important in promoting association of the FMN domain with the oxygenase domain.

CaMNN and CaMCC could be bound to either of the two EF hand pairs in position to recognize their preferred targets and with the other pair unassociated with the binding site. However, it is likely that the other EF hand pair often fills the position occupied by the opposite EF hand pair in wild-type CaM. For example, CaMNN would bind to the N-terminal N-type pair associated with the N-type recognition site at the C-terminus of the target. Meanwhile, the C-ter- minal N-type pair would weakly associate with the C-type recognition site at the N-terminus of the target. Single molecules of CaMNN and CaMCC can thus in principle occupy the entire CaM binding site, albeit at lower affinity.

Conversely, electron transfer through the reductase domain of eNOS to cytochrome c was stimulated by constructs with two EF hand pairs joined by the cen- tral linker region, although CaMNN, which contains the N-terminal EF hand pair of CaM, was the most c effective. nCaM slightly stimulated cytochrome reduction. As both CaMNN and CaMCC are capable of promoting electron transfer to cytochrome c, it appears that CaM requirements for promotion of FMN domain release in eNOS are not as stringent as in nNOS [13]. Although the patterns of cNOS activa- tion by the mutant CaM constructs are similar, differ- ences in the relative importance of the elements of CaM reveal underlying differences between nNOS and eNOS.

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CaMNN was the only CaM protein that showed appreciable •NO production rates with nNOS and eNOS (Table 1), which correlates well with strong binding to the NOS CaM binding domains (Fig. 3). All other mutant CaM proteins failed to either activate or bind the cNOS enzymes. These results indicate that the N-terminal domain in conjunction with at least the central linker region is required for binding and •NO Owing to the high susceptibility of iNOS to proteo- lysis during purification, coupled with the enzyme’s strong binding to wild-type CaM, studies on the mech- anism of CaM’s ablilty to promote electron transfer within the iNOS homodimer have been limited. As in our previous work [13], studies of the role of different

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electron transfer from FMN to the heme at lower Ca2+ concentrations suggest that these constructs are better at promoting FMN domain release than FMN domain–oxygenase association. This trend does not extend to iNOS coexpressed with CaMNN, as there was no significant decrease in NADPH oxidative activ- ity at high or low Ca2+ concentrations. This indicates that two EF hand pairs joined by the central linker region in combination with the N-terminal EF hand pair of CaM is sufficient to maintain NADPH oxida- tion activity in the presence or absence of Ca2+.

EF hand pairs of CaM in iNOS activation necessitated the development of separate coexpression systems for each of the mutant CaM proteins. Our results using the enzymes bound to the CaM proteins showed signi- ficant differences in the role of CaM activating iNOS when contrasted with the cNOS enzymes (compare Tables 1 and 2). Similar results were recently reported using a coexpression method consisting of iNOS and different Drosophila CaM proteins with mutations in each of the Ca2+-binding sites of CaM [28]. Their results further support our previous findings that EF hands 2 and 3 are important for iNOS–CaM activa- tion. Coexpression with each of the CaM proteins did not significantly affect electron transfer through the reductase domains to the cytochrome c. These results are consistent with our previous study demonstrating that electron transfer within the reductase domain of iNOS is CaM-independent using only the reductase domains of both human and mouse iNOS [29].

The coexpression studies of nCaM and CaMNN with iNOS both displayed reproducibly higher rates of cytochrome c reduction in the presence and absence of Ca2+ compared with wild-type CaM (Table 2). The increased rates observed with nCaM and CaMNN may indicate that these constructs produce a higher yield of enzyme in which the FMN domain is exposed to cytochrome c rather than shielded by interactions with the rest of the reductase complex or the oxyge- nase domain, suggesting that they are better at promo- ting release than reassociation.

interactions

The iNOS enzyme coexpressed with CaM is a highly stable complex; removal of free Ca2+ from the system has little effect on enzyme stability. However, Ca2+ che- lation does affect •NO production by iNOS through a conformational change within the N-terminal domain of CaM. In contrast, iNOS coexpressed with each of the CaM mutants is less stable than iNOS coexpressed with wild-type CaM. This is apparent from the gel filtration (Fig. 2) and gel mobility shift assays (Fig. 3) when iNOS coexpressed with wild-type CaM is compared with the mutant CaM proteins. In the presence of EDTA, the enzyme coexpressed with mutant CaM protein loses all detectable activity and appears to aggregate. The addi- tion of exogenous CaM to these samples at the same time as EDTA protects the enzyme based on enzyme activity assays (Table 2) and apparently maintains the dimeric structure of the enzyme (Fig. 2F).

In contrast with the results obtained for nNOS and eNOS, we find that nCaM is just as effective in pro- moting •NO production as CaMNN when bound to iNOS. This result was surprising as nCaM only con- sists of the N-terminal EF hand pair with no central linker region. The requirement of the N-terminal EF hand pair of CaM for activation of •NO production by iNOS suggests that this structure is vital in promo- ting FMN–oxygenase (Table 2). This result is consistent with our previous study showing the importance of EF hand 2 of CaM in binding and activating the iNOS enzyme in the presence and absence of Ca2+ [13]. It is noteworthy that iNOS coex- pressed with CaMCC produces •NO at 50% of the rate of iNOS coexpressed with CaM. This may be caused by a tethering effect, in which the central linker region orients the N-terminal domains of CaMCC into a conformation capable of promoting a reduced level of •NO production [4]. The addition of excess EDTA to the assays resulted in significant decreases in •NO production rates by iNOS coexpressed with all of the mutant CaM proteins but not with wild-type CaM.

to note that it

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The iNOS enzyme coexpressed with wild-type CaM showed no notable difference in NADPH oxidation rates in the presence of higher and lower concentra- tions of Ca2+, which is expected as the affinity of iNOS for CaM is very strong even in the presence of 10 mm EGTA [30]. Although NADPH oxidation rates for iNOS coexpressed with nCaM, cCaM and CaMCC in the presence of Ca2+ and EDTA do not correlate well with the corresponding •NO production rates, addition of EDTA significantly decreased both NADPH consumption and •NO production stimulated by these constructs (Table 2). The very low rates of Aggregation and activity measurements indicate that the effects of EDTA incubation are partially reversible by Ca2+ addition, suggesting that nCaM rebinds and may not even be fully dissociated. By these criteria, wild-type CaM is more efficient at reversing the EDTA- induced aggregation of iNOS at higher Ca2+ concentra- tions. However, this is important observed aggregation effect may be concentration- dependent. Gel filtration, native PAGE and CD studies are performed at micromolar concentrations compared with nanomolar concentrations used during kinetics; it is conceivable that the 1000-fold lower concentration of iNOS under the assay conditions used may produce significant differences in aggregation behavior.

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Calmodulin domain activation of NOS

1 8 14 eNOS TRKKTFKEVANAVKISASLMGTLM nNOS RRAIGFKKLAEAVKFSAKLMGGAM iNOS RREIPLKVLVKAVLFACMLMRKTM

Fig. 4. Alignment of CaM-binding domain sequences of the three NOS isoforms. The amino acids in bold and numbered are con- served in the 1-8-14 CaM-binding motif. The amino-acid residues underlined are described in the Discussion section.

in this

Using the published co-ordinates for the structure of an eNOS peptide bound to CaM [21], a model shown in Fig. 5 was created consisting of the eNOS peptide bound to only the first 75 residues of CaM to repre- sent the nCaM construct. Our CD results indicate that the peptide forms a helical structure when bound to nCaM (results not shown). As shown in Fig. 5, most of the helical target site including the predicted binding site of the N-terminal EF hand pair of CaM is shiel- ded from solvent. Complete or partial dissociation of nCaM in the presence of EDTA should expose hydro- phobic residues region. The CaM-binding domain of iNOS has greater hydrophobic character than the cNOS enzymes, which may in part account for the increased affinity of iNOS peptides for CaM in the presence of EDTA. The model in Fig. 5 shows

Table 3. Comparison of eNOS and iNOS CaM-binding domains predicted to aggregate by TANGO with CaM residues that interact with the peptide residues. Amino-acid residues in CaM shown to be within 4 A˚ of the eNOS CaM-binding peptide reported in [21]. Amino acids in bold represent conserved residues in the respective CaM-binding domains.

CaM-binding domain

CaM amino acids in contact with eNOS peptide

eNOS

iNOS

N-terminal domain

C-terminal domain

Central helix

Glu11, Glu14, Ala15, Leu18

Val91, Phe91, Leu112 Ser81, Glu84, Ile85, Ala88, Met145

Met76

Glu87 Glu84

Met76

Glu11, Phe12, Ala15, Met72 Ala15, Leu18, Phe19, Leu39 Leu39 Met72, Lys75 Phe19, Met36, Met51, Met71, Met72, Lys75

A502 V503 K504 I505 S506 A507 S508 L509

A521 V522 L523 F524 A525 C526 M527 L528

The statistical mechanics algorithm TANGO pre- dicts the propensity of peptides and proteins to aggre- gate [31]. Under conditions used in our experiments, the iNOS CaM-binding peptide has a very high pro- pensity for aggregation (AGG value ¼ 338.19) com- pared with eNOS (AGG value ¼ 1.00) and nNOS (AGG value ¼ 0). The TANGO results also suggest that the iNOS CaM-binding sequence ‘AVLFACML’ is particularly susceptible to aggregation (Fig. 4). On the basis of the CaM–eNOS peptide structure [21], the N-terminal domain residues of CaM would be expec- ted to predominantly interact with this region of the iNOS CaM-binding peptide (Table 3). that eNOS residue K504 is exposed to the solvent. The alignment of the CaM-binding sequences in the three NOS isoforms shown in Fig. 4 indicate that the lysine residue found at this position in both cNOS enzymes is a hydro- phobic leucine residue in iNOS. The exposure of a the hydrophobic region upon helix formation of

A

B

Fig. 5. Structures of nCaM bound to eNOS peptide. (A) Structure from the perspective of looking down the eNOS peptide barrel, and (B) a perpendicular representation of (A). Structures are derived from the PDB 1NIW [21]. CaM residues 4–75 peptide backbone, Ca2+ ions, and eNOS peptide backbone are shown in red, yellow, and blue, respectively. Peptide residues V503, K504, and A507 are shown in grey. Struc- tures were visualized using WebLab Viewer- Lite (Accelrys).

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Experimental procedures

CaM protein subcloning

pnCaMChlor

The chloramphenicol-resistant CaM expression vector pCaMChlor was a gift from A. Persechini (University of Missouri-Kansas City, MO, USA). Introduction of a stop codon at residue 76 and a reporter XbaI cut site by PCR mutagenesis produced pnCaMChlor, encoding the N-ter- minal residues 1–75. The forward and reverse primers were:

pC76STF, 5¢-ATGGCGAGGAAGATGTAATCTAGAG

ACACGGACAGCGAAG-3¢

pC76STR, 5¢-CTTCGCTGTCCGTGTCTCTAGATTAC

ATCTTCCTCGCCAT-3¢.

the

pnCaMChlor was verified by sequencing and used for

coexpression with iNOS.

iNOS peptide could account for the tendency of these peptides to aggregate. Our gel mobility shift assays indicate that two nCaM molecules bind to each iNOS peptide (Fig. 3). In the presence of excess peptide, the nCaM protein does not enter the gel and appears to aggregate. At low peptide to CaM ratios, the first nCaM protein must bind to the normal site on the peptide while second nCaM likely interacts weakly at another site on the peptide further shield- ing it from the solvent. Upon the addition of excess peptide, the weakly bound nCaM is displaced to bind the freshly added peptide. This displacement exposes hydrophobic regions of the iNOS peptide including the aforementioned leucine residue resulting in a pro- cess that may lead to aggregation.

pcCaMKan, coding for residues 76–148, was used for coex- pression with iNOS. The coding region was PCR amplified, introducing unique flanking NcoI and EcoRI sites for sub- cloning into a vector suitable for coexpression. Primers used were:

pcCaMKan

cCaMNcoI53,

5¢-CGATGATGGCGAGGACCATGGA

GGACACGGACAGCG-3¢

cCaMEcoRI35, 5¢-TGCATGATAAAGAAGGAATTCA

TAAGTGCGGCGA-3¢.

Studies using modified CaM constructs have shown that the requirements for activation of CaM-stimulated enzymes vary greatly [4,14,32–34]. Our results are novel as the N-terminal domain of CaM alone is suffi- cient to activate the iNOS isozyme to 70% maximal activity in the presence of Ca2+. Although this is not the first time that a single domain of CaM has been reported to activate its target protein, it is unique in that iNOS is 70% active at stoichiometric concentra- tions of nCaM. In addition, our results provide insight

The PCR product was blunt end ligated into the SrfI site of pPCR-SCRIPT Amp SK(+). The pcCaMPCRscript vec- tor was subsequently digested with NcoI and EcoRI and subcloned into the kanamycin-resistant pET28a vector (Novagen, Madison, WI, USA) cut with the same enzymes; pcCaMKan was verified by sequencing.

into the CaM requirement for iNOS expression. iNOS cleavage at the CaM binding site had been reported previously, but the aggregation phenomenon observed here in vitro suggests that, in the absence of coexpressed CaM, the enzyme aggregates, forming inclusion bodies and greatly reducing protein yield.

The vectors pCaMNNAmp and pCaMCCAmp, coding for CaMNN (residues 1–81, followed by 9–75) and CaMCC (residues 1–8, 82–148, 76–81, followed by 82–148), were a gift from A. Persechini [4]. Their ampicillin resistance neces- sitated construction of new vectors for coexpression with iNOS. Coding regions for CaMNN and CaMCC were sub- cloned into the kanamycin-resistant pET9dCaM plasmid consisting of a pET9d vector (Novagen) carrying rat cal- modulin that has unique flanking NcoI and PstI restriction sites. The products, pCaMNNKan and pCaMCCKan, were verified by sequencing.

pCaMNNKan and pCaMCCKan

Expression and purification of CaM protein

Overnight cultures of transformed BL21 (DE3) Escherichia coli were used to inoculate 1 L Luria–Bertani medium in 4-L

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A recent paper reported that mammalian cells may regulate iNOS by removing misfolded and aggregated proteins by a pathway that leads to the formation of aggresomes [35]. This may provide a rapid means of clearing the cells of iNOS that would be detrimental to the cell because of its prolonged production of large amounts of •NO. Our finding that displacement of CaM from iNOS leads to aggregation provides a possible mechanism for the regulation of the enzyme. The total intracellular concentration of CaM in the cell appears to be significantly below the total con- centration of its targets, making it a limiting factor in their regulation [36]. In the dynamic environment of the cell, the network of CaM-dependent signaling pathways may play a role in the cellular regulation of protein processing. Excess production of iNOS in the absence of sufficient quantities of CaM may lead to the aggregation of the enzyme and ultimately its disposal. Future cell culture studies are planned to further explore this possibility in vivo.

D. E. Spratt et al.

Calmodulin domain activation of NOS

described previously [13,29]. Quadruplicate reactions were initiated by NADPH addition and monitored on a 96-well plate reader at 25 (cid:1)C. Reaction mixtures contained 5.5 nm iNOS, 5.5 nm nNOS or 50 nm eNOS and 2 lm CaM or mutant CaM protein in a final volume of 100 lL.

flasks supplemented with 100 lgÆmL)1 appropriate anti- biotics. The 1-L cultures were grown at 37 (cid:1)C to A600 of 0.8– 1.2, induced with 500 lm isopropyl b-d-thiogalactoside and harvested after 3 h of expression. Cells were harvested, fro- zen, and stored at )80 (cid:1)C. Cells were thawed on ice, resus- pended in 4 vol. 50 mm Mops, pH 7.5, containing 100 mm KCl, 1 mm EDTA and 1 mm dithiothreitol, and homogen- ized using an Avestin EmulsiFlex-C5 homogenizer (Ottawa, ON, Canada). CaM was purified as previously described [13], frozen in aliquots on dry ice, and stored at )80 (cid:1)C. Electrospray ionization MS was performed on purified pro- teins using a quadrupole time-of-flight spectrometer (Micro- mass, Manchester, UK) with an internal standard [26].

Gel filtration studies

The dimerization of iNOS coexpressed with native CaM and nCaM was evaluated by gel filtration on a Superdex 200 HR column (Amersham Biosciences), equilibrated with TGND buffer (50 mm Tris ⁄ HCl, pH 7.5, 10% glycerol, 0.1 m NaCl, and 1 mm dithiothreitol) when observing the effect of basal Ca2+ concentrations, and equilibrated with TGND buffer plus 250 lm EDTA to chelate endogenous Ca2+. The flow rate was maintained at 0.5 mLÆmin)1 with an AKTApurifier System for Chromatography (Amersham Biosciences, Baie d’Urfe, PQ, Canada), and the temperature was kept constant at 7 (cid:1)C with an electronic wine cooler (Sylvania, Markham, ON, Canada). Eluted protein and heme were detected at 280 and 398 nm, respectively, with a flow-through detector. A gel filtration standard kit (Sigma, Oakville, ON, Canada) containing multiple molecular mass markers was used to calibrate the column.

Rat neuronal and bovine endothelial NOS were expressed in E. coli and purified as previously described [13,26,37]. Human iNOS carrying a deletion of the first 70 amino acids and an N-terminal polyhistidine tail was coexpressed with CaM or a CaM mutant in BL21 (DE3) E. coli. This pro- tein, which will be referred to as iNOS, was purified using ammonium sulfate precipitation, metal chelation chroma- tography, and 2¢,5¢-ADP affinity chromatography as previ- ously reported [13,37].

Expression and purification of NOS enzyme

The initial rate of •NO synthesis was measured using the spectrophotometric oxyhemoglobin assay as previously des- cribed [13,26,37,38]. Assays were performed at 25 (cid:1)C in a SpectraMax 190 96-well UV-visible spectrophotometer using Soft Max Pro software (Molecular Devices, Sunny- vale, CA, USA). eNOS, nNOS and iNOS were assayed at concentrations of 70, 30 and 28.5 nm, respectively, in 100- lL total well volumes. Unless otherwise stated, 200 lm CaCl2 or 250 lm EDTA and 2 lm wild-type CaM or mutant CaM protein were added to the appropriate samples.

Kinetics CaM mobility shift assay with synthetic NOS peptides Oxyhemoglobin assay

NADPH oxidation by NOS was monitored at 340 nm (e ¼ )0.0152 A unit per nmol) as previously described [13,26]. Quadruplicate reactions were initiated by l-arginine addition and monitored on a 96-well plate reader at 25 (cid:1)C. Reaction mixtures contained 49 nm iNOS, 70 nm nNOS or 100 nm eNOS and 2 lm CaM or mutant CaM protein in a final volume of 100 lL.

NADPH oxidase activity

The NOS CaM-binding domain peptides for bovine eNOS (TRKKT FKEVA NAVKI SASLM; residues 493–512), rat nNOS (KRRAI GFKKL AEAVK FSAKL MGQ; residues 725–747) and human iNOS (RPKRR EIPLK VLVKA VLFAC MLMRK; residues 507–531) were synthesized by SynPeP (SynPeP Corporation, Dublin, CA USA). The abil- ity of CaM and CaM mutant proteins to bind the synthetic NOS peptides was determined from the relative mobility shift of CaM in the presence of each peptide [39]. In a total volume of 15 lL, CaM, CaMNN, and CaMCC (20 lm) were incubated with increasing molar ratios of peptide to CaM protein (0, 0.25, 0.5, 0.75, 1, 1.5, 2, 4, 6, and 8) in (100 mm Tris ⁄ HCl, pH 7.5, with either binding buffer 0.2 mm CaCl2 or 1 mm EDTA) at room temperature for 1 h. nCaM and cCaM (60 lm) were incubated with increas- ing molar ratios of peptide to CaM protein (0, 0.125, 0.25, 0.375, 0.5, 0.75, 1, 2, 4, and 8) using the same conditions as CaM. After 1 h, volumes of the samples were halved by the sample loading buffer consisting of 50% glycerol with bromophenol blue as a tracer. The mixtures were then elec- trophoresed on 15% nondenaturing polyacrylamide gels containing 0.375 m Tris ⁄ HCl, pH 8.8, 4 m urea, and either 0.2 mm CaCl2 or 1 mm EDTA. Gels were run at a constant voltage of 100 V in electrode running buffer, which consis- ted of 25 mm Tris ⁄ HCl, pH 8.3, 192 mm glycine, 4 m urea, and either 0.2 mm CaCl2 or 1 mm EDTA. The gels were

The NADPH-dependent reduction of cytochrome c was monitored at 550 nm (e ¼ 0.0488 A unit per nmol) as

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Cytochrome c reductase activity

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then stained and visualized using Coomassie Brilliant Blue R-250.

12 Cho HJ, Xie QW, Calaycay J, Mumford RA, Swiderak KM, Lee TD & Nathan C (1992) Calmodulin is a sub- unit of nitric oxide synthase from macrophages. J Exp Med 176, 599–604.

Acknowledgements

13 Newman E, Spratt DE, Mosher J, Cheyne B, Mont- gomery HJ, Wilson DL, Weinburg JB, Smith SME, Salerno JC, Ghosh DK et al. (2004) Differential activa- tion of nitric-oxide synthase isozymes by calmodulin- troponin C chimeras. J Biol Chem 279, 33547–33557. 14 Persechini A, McMillan K & Leakey P (1994) Activa- tion of myosin light chain kinase and nitric oxide synthase activities by calmodulin fragments. J Biol Chem 269, 16148–16154.

We thank Dr Anthony Persechini for providing the plasmids coding for CaM, CaMNN and CaMCC, Dr Art Szabo for providing the synthetic NOS CaM- binding domain peptides, and Bo Cheyne for technical assistance. The present work was supported by Grant 183521 to JGG from the Natural Sciences and Engi- neering Research Council of Canada.

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