THE RABBIT B-CELL ANTIGEN RECEPTOR IS NONCOVALENTLY ASSOCIATED WITH UNIQUE HETEROMERIC PROTEIN COMPLEXES - POSSIBLE INSIGHTS INTO THE MEMBRANE IGM IGD COEXPRESSION PARADOX/

We describe several proteins that are components of the rabbit B cell receptor complex. Two proteins (37 kDa and 42 kDa) were found in non-c ovalent association with IgM expressed on B cells from peripheral bloo d. These proteins were also immunoprecipitated by anti-B29 (Ig-beta) a nd anti-mbl (Ig-alpha) monoclonal antibodies. As in the mouse and huma n, the IgM associated molecules were found as heteromeric structures w ith non-reduced apparent molecular weights of approximately 70-75 kDa. On rabbit B cells we also found these proteins in a 100-135 kDa compl ex which may represent trimeric or tetrameric structures. By Western b lot, the 37 kDa protein was identified as rabbit Ig-beta (B29), sugges ting that the 42 kDa protein is rabbit Ig-alpha. These data suggest th at rabbit IgM is associated with both Ig-alpha/beta and Ig-(alpha beta )(2) or alpha beta gamma complexes. When similar immunoprecipitation s tudies were performed on lysates made from B cells isolated from appen dix follicles, we found two additional IgM associated protein complexe s containing 34 kDa and 36 kDa proteins.