Distinct domains within yeast Sec61p involved in post-translational translocation and protein dislocation

The translocation of secretory polypeptides into and across the membrane of the endoplasmic reticulum (ER) occurs at the translocon, a pore-forming st ructure that orchestrates the transport and maturation of polypeptides at t he ER membrane. Recent data also suggest that misfolded or unassembled poly peptides exit the ER via the translocon for degradation by the cytosolic ub iquitin/proteasome pathway. Sec61p is a highly conserved multispanning memb rane protein that constitutes a core component of the translocon, We have f ound that the essential function of the Saccharomyces cerevisiae Sec61p is retained upon deletion of either of two internal regions that include trans membrane domains 2 and 3, respectively, However, a deletion mutation encomp assing both of these domains was found to be nonfunctional, Characterizatio n of yeast mutants expressing the viable deletion alleles of Sec61p has rev ealed defects in post-translational translocation. In addition, the transme mbrane domain 3 deletion mutant is induced for the unfolded protein respons e and is defective in the dislocation of a misfolded ER protein. These data demonstrate that the various activities of Sec61p can be functionally diss ected, In particular, the transmembrane domain 2 region plays a role in pos t-translational translocation that is required neither for cotranslational translocation nor for protein dislocation.