Independent Trafficking of Ig-α/Ig-β and μ-Heavy Chain Is Facilitated by Dissociation of the B Cell Antigen Receptor Complex

The BCR relays extracellular signals and internalizes Ag for processing and presentation. We have previously demonstrated that ligation of the BCR destabilizes Ig-alpha/Ig-beta (Ig-alphabeta) from mu-H chain (mum). In this study we report that receptor destabilization represents a physical separation of mum from Ig-alphabeta. Sucrose gradient fractionation localized Ig-alphabeta to G(M1)-containing lipid microdomains in the absence of mum. Confocal and electron microscopy studies revealed the colocalization of unsheathed mum with clathrin-coated vesicles. Furthermore, mum failed to associate with clathrin-coated vesicles when receptor destabilization was inhibited, suggesting that unsheathing of mum is required for clathrin-mediated endocytosis. In summary, we found that Ag stimulation physically separates Ig-alphabeta from mum, facilitating concomitant signal transduction and Ag delivery to the endocytic compartment.