The type III secretion system (T3SS) is a complex macromolecular machin-ery employed by a number of Gram-negative species to initiate infection.
Toxins secreted through the system are synthesized in the bacterial cyto-plasm and utilize the T3SS to pass through both bacterial membranes and
the periplasm, thus being introduced directly into the eukaryotic cytoplasm.
Most integral membrane proteins are targeted, inserted and assembled in
the endoplasmic reticulum membrane. The sequential and potentially over-lapping events necessary for membrane protein integration take place at
sites termed translocons, which comprise a specific set of membrane pro-teins acting in concert with ribosomes and, probably, molecular chaperones
to ensure the success of the whole process.
Type III secretion systems of Gram-negative pathogenic bacteria allow the
injection of effector proteins into the cytosol of host eukaryotic cells. Cros-sing of the eukaryotic plasma membrane is facilitated by a translocon, an
oligomeric structure made up of two bacterial proteins inserted into the
host membrane during infection.
Most of the estimated 1000 or so chloroplast proteins are synthesized as
cytosolic preproteins with N-terminal cleavable targeting sequences (transit
peptide). Translocon complexes at the outer (Toc) and inner chloroplast
envelope membrane (Tic) concertedly facilitate post-translational import of
preproteins into the chloroplast.
Twin-arginine translocation (Tat)-mediated protein trans-port across the bacterial cytoplasmicmembrane occurs only
after synthesis and folding of the substrate protein that
contains a signal peptide with a characteristic twin-arginine
motif. This implies that premature contact between the Tat
signal peptide and the Tat translocon in themembranemust
be prevented. We used site-specific photo-crosslinking to
demonstrate that the signal peptide of nascent Tat proteins
is in close proximity to the chaperone and peptidyl-prolyl
isomerase trigger factor (TF). ...
Higher plant chloroplasts possess at least four different pathways for pro-tein translocation across and protein integration into the thylakoid mem-branes. It is of interest with respect to plastid evolution, which pathways
have been retained as a relic from the cyanobacterial ancestor (‘conserva-tive sorting’), which ones have been kept but modified, and which ones
were developed at the organelle stage, i.e. are eukaryotic achievements as
(largely) the Toc and Tic translocons for envelope import of cytosolic pre-cursor proteins. ...