Biosynthesis of inositol trisphosphate receptors: selective association with the molecular chaperone calnexin

Abstract

A prominent labelled polypeptide having the same mobility as type-I inositol trisphosphate receptor (IP(3)R) was immunoprecipitated from WB-cell lysates by antibodies to calnexin, an ER integral membrane chaperone. The identity of this polypeptide was confirmed by re-immunoprecipitation of the radioactive polypeptides released from calnexin-antibody immunoprecipitates with type-I IP(3)R antibody. The interaction of calnexin with newly synthesized type-I IP(3)R was transient and inhibited by treatment of the cells with dithiothreitol or the glucosidase inhibitor N-methyldeoxynojirimicin. In similar experiments, there was no evidence for the binding of type-I IP(3)R to calreticulin, an ER luminal chaperone. Calnexin (but not calreticulin) associated with newly synthesized FLAG (DYKDDDDK epitope)-tagged type-III IP(3)R expressed in COS-7 cells. In order to further define the mechanism of interaction of nascent IP(3)R with chaperones, we have utilized an in vitro rabbit reticulocyte translation assay programmed with RNA templates encoding the six putative transmembrane (TM) domains of IP(3)Rs. In accordance with the known preference of calnexin for monoglucosylated oligosaccharide chains, calnexin antibody preferentially immunoprecipitated a proportion of glycosylated type-I translation product. Addition of glucosidase inhibitors prevented the association of calnexin with in vitro translated type-I TM construct. Using truncated RNA templates we found that calnexin did not associate with the first four TM domains but retained affinity for the construct encoding TM domains 5 and 6, which contains the glycosylation sites. We propose that calnexin is a key chaperone involved in the folding, assembly and oligomerization of newly synthesized IP(3) receptors in the ER.