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Review
. 2011 Jun;68(12):2053-66.
doi: 10.1007/s00018-011-0681-y. Epub 2011 Apr 10.

SecA, a remarkable nanomachine

Ilja Kusters  1 , Arnold J M Driessen
Affiliations
Review

SecA, a remarkable nanomachine

Ilja Kusters et al. Cell Mol Life Sci. 2011 Jun.

Abstract

Biological cells harbor a variety of molecular machines that carry out mechanical work at the nanoscale. One of these nanomachines is the bacterial motor protein SecA which translocates secretory proteins through the protein-conducting membrane channel SecYEG. SecA converts chemically stored energy in the form of ATP into a mechanical force to drive polypeptide transport through SecYEG and across the cytoplasmic membrane. In order to accommodate a translocating polypeptide chain and to release transmembrane segments of membrane proteins into the lipid bilayer, SecYEG needs to open its central channel and the lateral gate. Recent crystal structures provide a detailed insight into the rearrangements required for channel opening. Here, we review our current understanding of the mode of operation of the SecA motor protein in concert with the dynamic SecYEG channel. We conclude with a new model for SecA-mediated protein translocation that unifies previous conflicting data.

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Figures

Fig. 1
Fig. 1
Crystal structures of SecYEG in top-view from the cytoplasm. SecY TMS 1–6 (red), TMS 7–10 (green), plug domain (blue), SecE (orange), SecG/β (yellow). a SecYEβ from M. jannaschii (PDB accession code: 1RH5). b SecYE from T. thermophilus co-crystallized with a Fab fragment (not shown) bound to the C5 loop of SecY (2ZJS). c SecYEG from T. maritima co-crystallized with SecA (not shown) (3DIN). d SecYE from P. furiosus. In the crystal, the C-terminal α-helix of a neighboring SecY molecule (not shown) inserts partially into the channel inducing opening of the lateral gate (3MP7)
Fig. 2
Fig. 2
Conformational changes in SecA crystal structures and SecA dimerization interfaces. a SecA protomer from B. subtilis (1M6N). b SecA from T. maritima co-crystallized with SecYEG (not shown) (3DIN). c Top view (cytoplasmic side) of the SecA–SecYEG co-crystal as in (b) with residues implicated in dimerization in dimeric SecA structures of E. coli (red, 2FSF), B. subtilis (green, 1M6N), T. thermophilus (orange, 2IPC) and M. tuberculosis (yellow, 1NL3). Residues SecY251 and SecA43 that were previously crosslinked [26] are shown as purple spheres. SecA is displayed in cyan and SecYEG in gray. d Side view of (c)
Fig. 3
Fig. 3
The reciprocating piston model of SecA-mediated protein translocation. The polypeptide chain of the preprotein is colored differently to illustrate distinct segments that are translocated in steps of defined size
See this image and copyright information in PMC

References

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