| Abstract | Secretion across the inner membrane in some Gram-negative bacteria occurs via the preprotein translocase
pathway. Proteins are produced in the cytoplasm as precursors, and require a chaperone subunit to direct them to
the translocase component. [ 2202721]. From there, the mature proteins are either targeted to the outer
membrane, or remain as periplasmic proteins. The translocase protein subunits are encoded on the bacterial
chromosome.
The translocase itself comprises 7 proteins, including a chaperone protein (SecB), an ATPase (SecA), an integral
membrane complex (SecCY, SecE and SecG), and two additional membrane proteins that promote the release of
the mature peptide into the periplasm (SecD and SecF) [2202721]. The chaperone protein SecB [11336818] is a highly acidic homotetrameric protein that exists as a "dimer of dimers" in the bacterial cytoplasm.
SecB maintains preproteins in an unfolded state after translation, and targets these to the peripheral membrane
protein ATPase SecA for secretion [10418149]. The structure of the Escherichia coli SecYEG assembly revealed a sandwich of two membranes
interacting through the extensive cytoplasmic domains [12167867]. Each membrane is composed of dimers of SecYEG. The
monomeric complex contains 15 transmembrane helices.
The eubacterial secY protein [1406280] interacts with the signal sequences of secretory proteins as well as with two other components of the protein translocation system: secA and secE. SecY is an integral plasma membrane protein of 419 to 492 amino acid residues that apparently contains 10 transmembrane (TM), 6 cytoplasmic and 5 periplasmic regions.
Cytoplasmic regions 2 and 3, and TM domains 1, 2, 4, 5, 7 and 10 are well conserved: the conserved cytoplasmic regions are believed to interact with cytoplasmic secretion factors, while the TM domains may participate in protein export [2110998]. Homologs of secY are found in archaebacteria [1764515]. SecY is also encoded in the chloroplast genome of some algae [1544427] where it could be involved in a prokaryotic-like protein export system across the two membranes of the chloroplast endoplasmic reticulum (CER) which is present in chromophyte and cryptophyte algae. |
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