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Review
. 2004 Nov;186(22):7467-73.
doi: 10.1128/JB.186.22.7467-7473.2004.

A little help from my friends: quality control of presecretory proteins in bacteria

Adam C Fisher  1 , Matthew P DeLisa
Affiliations
Review

A little help from my friends: quality control of presecretory proteins in bacteria

Adam C Fisher et al. J Bacteriol. 2004 Nov.
No abstract available

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Figures

FIG. 1.
FIG. 1.
(A) Schematic of Sec translocation. Briefly, (a) SecB binding of a nascent polypeptide maintains export competence and assists in proper targeting to the Sec machinery. SecA serves several functions, including (b) preprotein binding; (c) targeting to the inner membrane; (not shown) maintaining quality control by assisting the cytoplasmic folding of nontransported polypeptides; and (d) driving preprotein translocation by repeated cycles of ATP-dependent membrane insertion-deinsertion. Finally, (e) translocation is completed and SecA and SecB are recycled. (B) Structural basis for Sec protein translocation adapted from the work of Van den Berg et al. (79) (see the text for a description).
FIG. 2.
FIG. 2.
Working model for Tat transport of folded proteins. Following preprotein folding in the cytoplasm (a and b), Tat substrates (S) are recognized by the translocon (c) in a process that likely involves TatB, TatC, and the leader peptide. According to the cyclical assembly model of Mori and Cline (54), preprotein binding to the TatB-TatC complex triggers assembly of multiple TatA monomers that likely form a translocation pore (d) through which a folded substrate is able to pass (e). Following successful transport, the TatABC complex disassembles. This model of assembly-disassembly may explain how the translocon can accommodate proteins of various sizes and how the Tat system can be present within membranes without compromising permeability to ions and protons.
FIG. 3.
FIG. 3.
Quality control of a nascent polypeptide during its voyage to the translocon. (a) The SRP targets nascent inner membrane proteins to the membrane by specifically recognizing transmembrane segments. On the other hand, (b) TF remains effectively bound to the mature region of nascent preproteins until a relatively late stage of translation. Following TF dissociation, cytosolic factors such as SecB and DnaK help to maintain preproteins in a loosely folded conformation. (c) SecA maintains quality control by assisting the cytoplasmic folding of nontransported polypeptides. Sec substrates that retain an extended conformation, such as through interaction with SecB (d), are efficiently transported. However, if prefolding of a Sec substrate occurs (e), the protein is degraded in the cytoplasm or else can become jammed in the translocon. For a subset of preproteins destined to the Tat translocon, association with a chaperone (f), such as DnaK or other Tat-specific factor, likely shields the signal sequence until folding is completed. This same factor or an additional factor may also promote correct folding and serve as a first layer of proofreading prior to translocation. Tat transport proceeds only if the Tat substrate is correctly folded; otherwise transport is aborted and the substrate is degraded by proteolytic machinery (g).
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References

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