BIP AND SEC63P ARE REQUIRED FOR BOTH CO- AND POSTTRANSLATIONAL PROTEIN TRANSLOCATION INTO THE YEAST ENDOPLASMIC-RETICULUM

Two interacting heat shock cognate proteins in the lumen of the yeast endoplasmic reticulum (ER), Sec63p and BiP (Kar2p), are required for posttranslational translocation of yeast a-factor precursor irt vitro. To investigate the role of these proteins in cotranslational translocation, we examined the import of invertase into wild-type, sec63, and kar2 mutant yeast membranes, We found that Sec63p and Kar2p are necessary for both co- and posttranslational translocation in yeast, Several kar2 mutants, one of which had normal ATPase activity, were defective in cotranslational translocation of invertase, We conclude that the requirement for BiP/Kar2p; which is not seen in a reaction reconstituted with pure mammalian membrane proteins [Gorlich, D. & Rapoport, T.A. (1993) Cell 75, 615-630], is not due to a distinction between cotranslational translocation in mammalian cells and posttranslational translocation in yeast cells.