Identification and characterization of a jem1p ortholog of candida albicans: Dissection of Jem1p functions in karyogamy and protein quality control in saccharomyces cerevisiae

Abstract

Jem1p of yeast Saccharomyces cerevisiae is a J-domain containing co-chaperone (J protein) in the endoplasmic reticulum (ER) lumen. Jem1p is required for nuclear fusion during mating (karyogamy) and functions together with another J protein, Scj1p, in protein folding and quality control in the ER as a partner for the ER Hsp70 (BiP/Kar2p). Candida albicans has a gene encoding a homolog of S. cerevisiae Jem1p, CaJem1p. CaJem1p localized in the ER when expressed in S. cerevisiae, and expression of CaJem1p from a single-copy plasmid suppressed the temperature sensitive growth and the ER quality control defect of the jem1{increment}scj1{increment} mutant, indicating that CaJem1p is functional in S. cerevisiae. However, CaJem1p suppressed the karyogamy defect of the jem1{increment} mutant only when it was over-expressed from a multicopy plasmid. Domain-swapping experiments showed that this was due to the difference between the N-terminal domains of ScJem1p and CaJem1p. The N-terminal domain of ScJem1p is essential for its function and interacts with Nep98p, a component of the spindle pole body involved in karyogamy. Since the interaction of CaJem1p with Nep98p is weaker than that of ScJem1p, the Nep98p-ScJem1p interaction is likely important for promoting karyogamy in S. cerevisiae. © 2008 by the Molecular Biology Society of Japan/Blackwell Publishing Ltd.