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J. Cell Biol.,
Volume 143, Number 4, November 16, 1998 921-933
* Department of Biochemistry and Molecular Biology, University of Massachusetts Medical School, Worcester, Massachusetts
01655-0103; and 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 Department of Biological Chemistry and Molecular Pharmacology and Dana Farber Cancer Institute, Harvard
Medical School, Boston, Massachusetts 02115
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 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.
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