The 160 N-terminal residues of calnexin define a novel region supporting viability in Schizosaccharomyces pombe.

Protein secretion is a complex process that can be modulated by folding factors in the endoplasmic reticulum (ER), such as calnexin, a highly-conserved molecular chaperone involved in quality control. In Schizosaccharomyces pombe, calnexin (Cnx1p) is essential for cell viability. The calnexin/Cnx1p determinants required for viability have been mapped within the last 123 residues of its C-terminus. To better understand the role(s) of calnexin/Cnx1p in secretion, we screened for cnx1 mutants 'super-secreting' cellulase. We identified ss14_cnx1, a mutant secreting 10-fold higher levels of the glycoprotein cellulase than the wild-type strain. While cellulase did not interact with ss14_Cnx1p, the ratio of secreted activity/quantity for this enzyme was not affected, suggesting that the quality control of folding in the ER was adequate in the mutant strain. Surprisingly, the ss14_Cnx1p mutant is composed of the 160 N-terminal amino acids of the mature molecule, thus this mutant defines a novel calnexin/Cnx1p region supporting Sz. pombe viability. Interestingly, like viable mutants spanning the last 52 aa of calnexin/Cnx1p, the 160 N-terminal residues encoded by ss14_cnx1 also forms a complex with the essential BiP chaperone. These results reveal the so far unidentified importance of the N-terminal region of calnexin/Cnx1p.