Identification and characterization of a Jem1p ortholog of Candida albicans: dissection of Jem1p functions in karyogamy and protein quality control in Saccharomyces cerevisiae.

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 jem1Deltascj1Delta mutant, indicating that CaJem1p is functional in S. cerevisiae. However, CaJem1p suppressed the karyogamy defect of the jem1Deltamutant 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.