SSS1 encodes a stabilizing component of the Sec61 subcomplex of the yeast protein translocation apparatus

The yeast SSS1 gene has been isolated as an extragenic high copy suppressor of sec61, a mutant displaying defects in protein translocation into the endoplasmic reticulum (ER). We found that SSS1 is an essential gene required for transfer of secretory precursors through the ER membrane. Here we demonstrate that the SSS1 product (Sss1p) is firmly bound to the ER membrane and exposes its amino-terminal half on the cytosolic side. Only detergent, or an alkali treatment, is effective at extracting Sss1p from the membrane. Coimmunoprecipitation experiments revealed that Sss1p and Sec61p participate in the same multisubunit complex. Cross-linking followed by immunoprecipitation specifically yielded an additional polypeptide of molecular mass 73 kDa. Moreover, Sss1p and Sec61p show mutually stabilizing interactions: Sss1p is destabilized in a sec61 mutant context, and mutated Sec61p is stabilized by Sss1p overproduction. These observations account for the isolation of SSS1 as a dosage-dependent suppressor of sec61. Since the polytopic integral membrane protein Sec61p is adjacent to translocating precursors and to ribosomes, and given the comparable translocation deficiencies of sss1 or sec61 mutants, we propose that Sss1p belongs to the "Sec61 subcomplex" that constitutes the pore of the membrane-bound translocation apparatus.