Evolutionary conservation of components of the protein translocation complex

PROTEIN translocation into the mammalian endoplasmic reticulum requires the Sec61p complex, which consists of three membrane proteins¹. The α-subunit, the homologue of Sec61p of yeast^2-4, shows some similarity to SecYp⁵, a key component of the protein export apparatus of bacteria^6,7. In Escherichia coli, SecYp is also associated with two other proteins (SecEp and band-1 protein)^8,9. We have now determined the sequences of the β- and γ-subunits of the mammalian Sec61p complex. Sec61-γ is homologous to SSSlp, a suppressor of sec61 mutants in Saccharomyces cerevisiae, and can functionally replace it in yeast cells. Moreover, Sec61-γ and SSSlp are structurally related to SecEp of E. coli and to putative homologues in various other bacteria. At least two sub-units of the Sec61/SecYp complex therefore seem to be key components of the protein translocation apparatus in all classes of organisms.