title: The 160 N-terminal residues of calnexin define a novel region supporting viability in Schizosaccharomyces pombe.
abstract: 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.
authors.affiliation: Department of Biochemistry, Université de Montréal, C.P. 6128, Succursale Centre-ville, Montréal, Québec H3C 3J7, Canada.
authors.forename: Fadi
authors.initials: F
authors.lastname: Hajjar
authors.forename: Pascale B
authors.initials: PB
authors.lastname: Beauregard
authors.forename: Luis A
authors.initials: LA
authors.lastname: Rokeach
mesh_terms: D020816:Amino Acid Motifs; D001230:Aspergillus / Q000201:enzymology / Q000235:genetics; D015139:Blotting, Southern; D037281:Calnexin / Q000235:genetics / Q000502:physiology*; D002480:Cellulase / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D004271:DNA, Fungal / Q000235:genetics; D005656:Fungal Proteins / Q000235:genetics / Q000502:physiology*; D015151:Immunoblotting; D008857:Microscopy, Interference; D016254:Mutagenesis, Insertional; D010957:Plasmids / Q000235:genetics; D016133:Polymerase Chain Reaction; D017510:Protein Folding; D012568:Schizosaccharomyces / Q000201:enzymology / Q000235:genetics / Q000502:physiology*
chemical_list: C094757:Cnx1 protein, S pombe; D004271:DNA, Fungal; D005656:Fungal Proteins; D037281:Calnexin; D002480:Cellulase