Sls1p Stimulates Sec63p-Mediated Activation of Kar2p in a Conformation-Dependent Manner in the Yeast Endoplasmic Reticulum

Abstract

ABSTRACT We previously characterized the SLS1 gene in the yeast Yarrowia lipolytica and showed that it interacts physically with Yl Kar2p to promote translocation across the endoplasmic-reticulum membrane (A. Boisramé, M. Kabani, J. M. Beckerich, E. Hartmann, and C. Gaillardin, J. Biol. Chem. 273:30903–30908, 1998). A Y. lipolytica Kar2p mutant was isolated that restored interaction with an Sls1p mutant, suggesting that the interaction with Sls1p could be nucleotide and/or conformation dependent. This result was used as a working hypothesis for more accurate investigations in Saccharomyces cerevisiae . We show by two-hybrid an in vitro assays that the S. cerevisiae homologue of Sls1p interacts with Sc Kar2p. Using dominant lethal mutants of Sc Kar2p, we were able to show that Sc Sls1p preferentially interacts with the ADP-bound conformation of the molecular chaperone. Synthetic lethality was observed between Δ Scsls1 and translocation-deficient kar2 or sec63-1 mutants, providing in vivo evidence for a role of Sc Sls1p in protein translocation. Synthetic lethality was also observed with ER-associated degradation and folding-deficient kar2 mutants, strongly suggesting that Sls1p functions are not restricted to the translocation process. We show that Sls1p stimulates in a dose-dependent manner the binding of Sc Kar2p on the lumenal J domain of Sec63p fused to glutathione S -transferase. Moreover, Sls1p is shown to promote the Sec63p-mediated activation of Kar2p's ATPase activity. Our data strongly suggest that Sls1p could be the first GrpE-like protein described in the endoplasmic reticulum.