InterPro domain: IPR005807

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 [ 1 ]. 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) [ 2 ]. The chaperone protein SecB [ 2 ] 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 [ 3 ]. SecE, part of the main SecYEG translocase complex, is ~106 residues in length, and spans the inner membrane of the Gram-negative bacterial envelope. Together with SecY and SecG, SecE forms a multimeric channel through which preproteins are translocated, using both proton motive forces and ATP-driven secretion. The latter is mediated by SecA. The structure of the Escherichia coli SecYEG assembly revealed a sandwich of two membranes interacting through the extensive cytoplasmic domains [ 4 ]. Each membrane is composed of dimers of SecYEG. The monomeric complex contains 15 transmembrane helices.

1. The sec and prl genes of Escherichia coli. J. Bioenerg. Biomembr. 22, 291-310
2. SecB, a molecular chaperone with two faces. Trends Microbiol. 9, 193-6
3. Effects of pre-protein overexpression on SecB synthesis in Escherichia coli. FEMS Microbiol. Lett. 176, 219-27
4. Three-dimensional structure of the bacterial protein-translocation complex SecYEG. Nature 418, 662-5