InterPro domain: IPR000185

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 ].

SecA is a cytoplasmic protein of 800 to 960 amino acid residues.Homologues of secA are also encoded in the chloroplast genome of some algae [ 4 ]as well as in the nuclear genome of plants [ 5 ]. It could be involved in theintraorganellar protein transport into thylakoids.

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. SecA is plastid-encoded in a red alga: implications for the evolution of plastid genomes and the thylakoid protein import apparatus. Mol. Gen. Genet. 236, 245-50
5. Isolation and characterization of the cDNA for pea chloroplast SecA. Evolutionary conservation of the bacterial-type SecA-dependent protein transport within chloroplasts. FEBS Lett. 364, 305-8