A 2ND TRIMERIC COMPLEX CONTAINING HOMOLOGS OF THE SEC6LP COMPLEX FUNCTIONS IN PROTEIN-TRANSPORT ACROSS THE ER MEMBRANE OF SACCHAROMYCES-CEREVISIAE

Yeast microsomes contain a heptameric Sec complex involved in post-tra nslational protein transport that is composed of a heterotrimeric Sec6 1p complex and a tetrameric Sec62-Sec63p complex, The trimeric Sec61p complex also exists as a separate entity that probably functions in co -translational protein transport, like its homolog in mammals, We have now discovered in the yeast endoplasmic reticulum membrane a second, structurally related trimeric complex, named Ssh1p complex, It consist s of Ssh1p (Sec Sixty-one homolog 1), a rather distant relative of Sec 61p, of Sbh2p, a homolog of the Sbh1p subunit of the Sec61p complex, a nd of Sss1p, a component common to both trimeric complexes, In contras t to Sec61p, Ssh1p is not essential for cell viability but it is requi red for normal growth rates, Sbh1p and Sbh2p individually are also not essential, but cells lacking both proteins are impaired in their grow th at elevated temperatures and accumulate precursors of secretory pro teins; microsomes isolated from these cells also exhibit a reduced rat e of posttranslational protein transport. Like the Sec61p complex, the Ssh1p complex interacts with membrane-bound ribosomes, but it does no t associate with the Sec62-Sec63p complex to form a heptameric Sec com plex, We therefore propose that it functions exclusively in the co-tra nslational pathway of protein transport.