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© The Rockefeller University Press, 0021-9525/1998//921 $5.00
The Journal of Cell Biology, Volume 143, Number 4, , 1998 921-933

Regular Articles

A Role for the DnaJ Homologue Scj1p in Protein Folding in the Yeast Endoplasmic Reticulum



Susana Silberstein*, Gabriel Schlenstedt{ddagger}, Pam A. Silver{ddagger}, and Reid Gilmore*

* Department of Biochemistry and Molecular Biology, University of Massachusetts Medical School, Worcester, Massachusetts 01655-0103; and {ddagger} Department of Biological Chemistry and Molecular Pharmacology and Dana Farber Cancer Institute, Harvard Medical School, Boston, Massachusetts 02115

Members of the eukaryotic heat shock protein 70 family (Hsp70s) are regulated by protein cofactors that contain domains homologous to bacterial DnaJ. Of the three DnaJ homologues in the yeast rough endoplasmic reticulum (RER; Scj1p, Sec63p, and Jem1p), Scj1p is most closely related to DnaJ, hence it is a probable cofactor for Kar2p, the major Hsp70 in the yeast RER. However, the physiological role of Scj1p has remained obscure due to the lack of an obvious defect in Kar2p-mediated pathways in scj1 null mutants. Here, we show that the {Delta}scj1 mutant is hypersensitive to tunicamycin or mutations that reduce N-linked glycosylation of proteins. Although maturation of glycosylated carboxypeptidase Y occurs with wild-type kinetics in {Delta}scj1 cells, the transport rate for an unglycosylated mutant carboxypeptidase Y (CPY) is markedly reduced. Loss of Scj1p induces the unfolded protein response pathway, and results in a cell wall defect when combined with an oligosaccharyltransferase mutation. The combined loss of both Scj1p and Jem1p exaggerates the sensitivity to hypoglycosylation stress, leads to further 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 mediate maturation of proteins in the RER lumen.

Key Words: glycosylation • endoplasmic reticulum • chaperones • heat-shock protein 70 • oligosaccharides

Abbreviations used in this paper: 5-FOA, 5-fluoro-orotic acid; AP, acid phosphatase; CPY, carboxypeptidase Y; Hsp70, heat shock protein 70 family; OST, oligosaccharyltransferase; ug-CPY, unglycosylated CPY; UPR, unfolded protein response.

Address all correspondence to Reid Gilmore, Department of Biochemistry and Molecular Biology, University of Massachusetts Medical School, 55 Lake Avenue North, Worcester, MA 01655-0103. Tel.: (508) 856-5894. Fax: (508) 856-6231. E-mail: reid.gilmore{at}banyan.ummed.edu

Dr. Schlenstedt's present address is Medizinische Biochemie, Gebaeude 44, Universitaet des Saarlandes, D66421 Homberg, Germany.


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