Abundance of Ssh1p modulates Sec61p stability and topogenesis

Abstract

Protein translocation across the endoplasmic reticulum (ER) membrane is mediated by translocons. Two translocons in the ER of Saccharomyces cerevisiae are the Sec61 and the Ssh1 ( Sec sixty‐one homolog 1) complexes. They contain Sec61p and Ssh1p as a pore‐forming core subunit, respectively, and share Sss1p as a complex stabilizing factor. Ssh1p is not essential for cell growth in yeast, and it remains unclear why yeast cells keep this non‐essential translocon component.Upon overexpression of Ssh1p, we observed a severe growth defect and rapid destabilization of Sec61p at high temperature. We reasoned that overexpressed Ssh1p, which is scarce in steady state might recruit Sss1p from the Sec61 complex, impairing its stability. To test this idea, co‐overexpression of Sss1p with Ssh1p was carried out and data show that it partially rescues a growth defect and Sec61 stability. Furthermore, overexpression of the Sss1p‐interaction‐defective mutant of ssh1p had no effect on growth or Sec61p stability. We also assessed synthesis of single‐spanning membrane proteins in SSH1 deletion and overexpression strains and found that Ssh1p is involved in biased topogenesis of single‐spanning membrane proteins.Our data characterized cellular roles of Ssh1p that it might contribute for □) adjusting Sec61p stability and □) mediating selective topologenesis of membrane proteins.