PROTEIN translocation into the mammalian endoplasmic reticulum requires the Sec61p complex, which consists of three membrane proteins(1). The alpha-subunit, the homologue of Sec61p of yeast(2-4), shows some similarity to SecYp(5), 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 beta- and gamma-subunits of the mammalian Sec61p complex. Sec61-gamma is homologous to SSS1p, a suppressor of sec61 mutants in Saccharomyces cerevisiae, and can functionally replace it in yeast cells. Moreover, Sec61-gamma and SSSIp are structurally related to SecEp of E. coli and to putative homologues in various other bacteria. At least two subunits of the Sec61/SecYp complex therefore seem to be keg components of the protein translocation apparatus in all classes of organisms.