Ssh1p Determines the Translocation and Dislocation Capacities of the Yeast Endoplasmic Reticulum

Sec61p is required both for protein translocation and dislocation across the membrane of the endoplasmic reticulum (ER). However, the cellular role of the Sec61p homolog Ssh1p has not been clearly defined. We show that Δssh1 mutant cells have strong defects in both SRP-dependent and -independent translocation. More-over, these cells were also found to be induced for the unfolded protein response and to be defective in dislocation of a misfolded ER protein. In addition, Δssh1 mutant cells rapidly became respiratory deficient. The other defects discussed above were suppressed in the respiratory-deficient state or under conditions where the rate of polypeptide translation was artificially reduced. These data identify Ssh1p as a component of a second, functionally distinct translocon in the yeast ER membrane.