Roles of calnexin and Ig-alpha beta interactions with membrane Igs in the surface expression of the B cell antigen receptor of the IgM and IgD classes.

Abstract

Membrane Igs (mIgs) of all five classes are associated with Ig-alpha beta dimers on the B cell surface. While mIgM requires the presence of these two associated molecules for its surface expression, mIgD does not. To study the structural basis for this differential Ig-alpha beta dependence, we created mutant mIgM and mIgD molecules (chimeras and those with reciprocal point mutations in their transmembrane sequences) and identified two amino acid residues in the transmembrane region of the mIg heavy chains responsible for this transport difference. Without Ig-alpha beta, mIgM and mutant mIgD molecules remained endoglycosidase H sensitive, consistent with endoplasmic reticulum (ER) localization. The molecular chaperone calnexin has previously been implicated in retaining unassembled mIgs in the ER. However, we found that inhibition of the association between calnexin and newly synthesized mIgs by castanospermine treatment did not lead to surface expression of normally retained mIgs. Therefore, calnexin cannot be the only retention molecule. Furthermore, for both wildtype and mutant mIgDs, association with calnexin was rather transient, thereby ruling out calnexin being a major ER retention molecule for mIgDs. Our study with castanospermine also showed that calnexin is required for wildtype mIgD surface expression only if Ig-alpha beta is absent, while the latter alone can function to promote mIg folding, assembly, and transport. Further study using our system will help to identify novel molecules and characterize their involvement in the control of mIg transport.