A high-molecular-weight complex of membrane proteins BAP29/BAP31 is involved in the retention of membrane-bound IgD in the endoplasmic reticulum

B cell antigen receptors (BCRs) are multimeric transmembrane protein complexes comprising membrane-bound immunoglobulins (mIgs) and Ig-α/Ig-β heterodimers. In most cases, transport of mIgs from the endoplasmic reticulum (ER) to the cell surface requires assembly with the Ig-α/Ig-β subunits. In addition to Ig-α/Ig-β, mIg molecules also bind two ER-resident membrane proteins, BAP29 and BAP31, and the chaperone heavy chain binding protein (BiP). In this article, we show that neither Ig-α/Ig-β nor BAP29/BAP31 nor BiP bind simultaneously to the same mIgD molecule. Blue native PAGE revealed that only a minor fraction of intracellular mIgD is associated with high-molecular-weight BAP29/BAP31 complexes. BAP-binding to mIgs was found to correlate with ER retention of chimeric mIgD molecules. On high-level expression in Drosophila melanogaster S2 cells, mIgD molecules were detected on the cell surface in the absence of Ig-α/Ig-β. This aberrant transport was prevented by coexpression of BAP29 and BAP31. Thus, BAP complexes contribute to ER retention of mIg complexes that are not bound to Ig-α/Ig-β. Furthermore, the mechanism of ER retention of both BAP31 and mIgD is not through retrieval from a post-ER compartment, but true ER retention. In conclusion, BAP29 and BAP31 might be the long sought after retention proteins and/or chaperones that act on transmembrane regions of various proteins.