Cell viability and secretion of active proteins in Schizosaccharomyces pombe do not require the chaperone function of calnexin

Folding of newly synthesized proteins within the ER (endoplasmic reticulum) is a rate-limiting step in protein secretion. Thus ER molecular chaperones and foldases have a major impact in determining the rate and yield of these crucial cellular processes. Calnexin is a key ER chaperone implicated in the folding, retention and targeting for degradation of proteins that go through the secretory pathway. Calnexin molecules contain a highly conserved central domain (hcd) that has been proposed to be involved in the interaction with folding substrates and other chaperones. To gain a better understanding of the roles played by calnexin in the secretory pathway, we examined the efficiency of fission yeast (Schizosaccharomyces pombe) strains expressing calnexin mutants to secrete different model proteins. Remarkably, calnexin hcd-deletion mutants, although devoid of detectable chaperone activity in vitro, confer viability and cause a considerable increase in the secretion of heterologous cellulase. Surprisingly the quality-control efficiency, measured as the activity/amount ratio of secreted model protein, was not severely reduced in these calnexin hcd-deletion mutant strains. Our results indicate that the essential function of calnexin does not reside in its role in the folding or in the retention of misfolded proteins. These observations suggest the existence of a highly stringent quality control mechanism in the ER of S. pombe that might reduce the secretion efficiency of endogenous proteins.