LHS1 and SIL1 provide a lumenal function that is essential for protein translocation into the endoplasmic reticulum

Lhs1p is an Hsp70-related chaperone localized in the endoplasmic reticulum (ER) lumen. Delta lhs1 mutant cells are viable but are constitutively induc ed for the unfolded protein response (UPR). Here, we demonstrate a severe g rowth defect in Delta ire1 Delta lhs1 double mutant cells in which the UPR can no longer be induced. In addition, we have identified a UPR-regulated g ene, SIL1, whose overexpression is sufficient to suppress the Delta ire1 De lta lhs1 growth defect. SIL1 encodes an ER-localized protein that interacts directly with the ATPase domain of Kar2p (BiP), suggesting some role in mo dulating the activity of this vital chaperone. SIL1 is a non-essential gene but the Delta lhs1 Delta sil1 double mutation is lethal and correlates wit h a complete block of protein translocation into the ER. We conclude that t he IRE1-dependent induction of SIL1 is a vital adaptation in Delta lhs1 cel ls, and that the activities associated with the Lhs1 and Sil1 proteins cons titute an essential function required for protein translocation into the ER . The Sil1 protein appears widespread amongst eukaryotes, with homologues i n Yarrowia lipolytica (Sls1p), Drosophila and mammals.