Calnexin Phosphorylation Attenuates the Release of Partially Misfolded α1-Antitrypsin to the Secretory Pathway

Calnexin is a type I integral membrane phosphoprotein resident of the endoplasmic reticulum. Its intraluminal domain has been deduced to function as a lectin chaperone coordinating the timing of folding of newly synthesized N-linked glycoproteins of the secretory pathway. Its C-terminal cytosolic oriented extension has an ERK1 phosphorylation site at Ser(563) affecting calnexin association with the translocon. Here we find an additional function for calnexin phosphorylation at Ser563 in endoplasmic reticulum quality control. A low dose of the mis-folding agent L-azetidine 2-carboxylic acid slows glycoprotein maturation and diminishes the extent and rate of secretion of newly synthesized secretory alpha 1-antitrypsin. Under these conditions the phosphorylation of calnexin is enhanced at Ser563. Inhibition of this phosphorylation by the MEK1 inhibitor PD98059 enhanced the extent and rate of alpha 1-antitrypsin secretion comparable with that achieved by inhibiting alpha-mannosidase activity with kifunensine. This is the first report in which the phosphorylation of calnexin is linked to the efficiency of secretion of a cargo glycoprotein.