InterPro domain: IPR002208

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 (SecY, 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 ]. 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.

The eubacterial secY protein [ 5 ] 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 [ 6 ]. Homologs of secY are found in archaebacteria [ 7 ]. SecY is also encoded in the chloroplast genome of some algae [ 8 ] 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.

This family consists of the protein translocase subunit SecY and protein transport protein Sec61 subunit alpha (Sec61a).

Sec61a is part of the Sec61 complex, which plays a crucial role in the insertion of secretory and membrane polypeptides into the ER. It is required for assembly of membrane and secretory proteins.

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
5. SecY and integral membrane components of the Escherichia coli protein translocation system. Mol. Microbiol. 6, 2423-8
6. Isolation of a secY homologue from Bacillus subtilis: evidence for a common protein export pathway in eubacteria. Mol. Microbiol. 4, 305-14
7. Presence of a gene in the archaebacterium Methanococcus vannielii homologous to secY of eubacteria. Biochimie 73, 683-8
8. A secY homologue is found in the plastid genome of Cryptomonas phi. FEBS Lett. 298, 93-6