Effects of calnexin deletion in Saccharomyces cerevisiae on the secretion of glycosylated lysozymes

Disruption of the calnexin gene in Saccharomyces cerevisiae did not lead to gross effects on the levels of cell growth and secretion of wild-type hen egg white lysozymes (HEWL). To investigate the function of calnexin in rela tion to the secretion of glycoproteins, we expressed both stable and unstab le mutant glycosylated lysozymes in calnexin-disrupted S. cerevisiae. The s ecreted amounts of stable mutant glycosylated lysozymes (G49N and S91T/G49N ) were almost the same in both wild-type and calnexin-disrupted S. cerevisi ae. In contrast, the secretion of unstable mutant glycosylated lysozymes (K 13D/G49N, C76A/G49N, and D66H/G49N) greatly increased in calnexin-disrupted S. cerevisiae, although their secretion was very low in the wild-type stra in. This indicates that calnexin may act in the quality control of glycopro teins. We further investigated the expression level of the mRNA of the mole cular chaperones BiP and PDI, which play a major role in the protein foldin g process in the ER, when glycosylated lysozymes were expressed in wild-typ e and calnexin-disrupted S. cerevisiae. The mRNA concentrations of BiP and PDI were evidently increased when the glycosylated lysozymes were expressed in calnexin-disrupted S. cerevisiae. This observation indicates that BiP a nd PDI may be induced by the accumulation of unfolded glycosylated lysozyme s due to the deletion of calnexin.