A ROLE FOR THE DNAJ HOMOLOG SCJ1P IN PROTEIN-FOLDING IN THE YEAST ENDOPLASMIC-RETICULUM

Members of the eukaryotic heat shock protein 70 family (Hsp70s) are re gulated by protein cofactors that contain domains homologous to bacter ial DnaJ. Of the three DnaJ homologues in the yeast rough endoplasmic reticulum (RER; Scj1p, Sec63p, and Jem1p), Scj1p is most closely relat ed to DnaJ, hence it is a probable cofactor for Kar2p, the major Hsp70 in the yeast RER. However, the physiological role of Scj1p has remain ed obscure due to the lack of an obvious defect in Kar2p-mediated path ways in scj1 null mutants. Here, we show that the Delta scj1 mutant is hypersensitive to tunicamycin or mutations that reduce N-linked glyco sylation of proteins. Although maturation of glycosylated carboxypepti dase Y occurs with wild-type kinetics in Delta scj1 cells, the transpo rt rate for an unglycosylated mutant carboxypeptidase Y (CPY) is marke dly reduced. Loss of Scj1p induces the unfolded protein response pathw ay, and results in a cell wall defect when combined with an oligosacch aryltransferase mutation. The combined loss of both Scj1p and Jem1p ex aggerates the sensitivity to hypoglycosylation stress, leads to furthe r induction of the unfolded protein response pathway, and drastically delays maturation of an unglycosylated reporter protein in the RER. We propose that the major role for Scj1p is to cooperate with Kar2p to m ediate maturation of proteins in the RER lumen.