Calnexin associates with Shaker K+ channel protein but is not involved in quality control of subunit folding or assembly.

Calnexin is part of an ER chaperone system that monitors and promotes the proper folding and assembly of glycosylated membrane proteins. To investigate the role of calnexin in the biogenesis of the voltage-dependent Shaker K+ channel, wild-type and mutant Shaker proteins were expressed in mammalian cells. Association with calnexin was assayed by coimmunoprecipitation. Calnexin interacted transiently with wild-type Shaker protein in the ER. In contrast, calnexin failed to associate with an unglycosylated Shaker mutant that makes active, cell surface channels. Therefore, glycosylation of Shaker protein is required for association with calnexin, but calnexin is not required for the proper folding and assembly of Shaker channels. We also investigated whether calnexin is involved in the ER retention of mutant Shaker proteins defective in subunit folding, assembly, or pore formation. Each of the mutant proteins associated transiently with calnexin during biogenesis. Calnexin dissociated from wild-type and mutant proteins with similar time courses. Thus, non-native Shaker proteins escape the folding sensor of the calnexin chaperone system. Furthermore, stable association with calnexin is not the mechanism by which these mutant proteins are retained in the ER. Our results indicate that calnexin is not involved in the quality control of subunit folding, assembly, or pore formation in Shaker K+ channels.