Structure, function, and biogenesis of SecY, an integral membrane protein involved in protein export

The E. coli secY (prlA) gene, located in the operator-distal part of the spc ribosomal protein operon, codes for an integral membrane protein, SecY. The phenotypes of temperature-sensitive and cold-sensitive mutations in secY suggest that the SecY protein plays an essential role in vivo to facilitate protein translocation, whereas the prlA mutations in this gene suggest that SecY may interact with the signal sequence of translocating polypeptides. SecY contains most probably six cytoplasmic and five periplasmic domains, as well as 10 transmembrane segments. Such membrane-embedded structure may confer the SecY protein a "translocator" function, in which it provides a protein-aceous pathway for passage of secreted as well as membrane proteins. Results obtained by in vitro analyses of the translocation reactions, as well as some new phenotypes of the secY mutants, are consistent with this notion. Possible interaction of SecY with other secretion and chaperone-like factors is also discussed.