ROLES OF HEAVY AND LIGHT-CHAINS IN IGM POLYMERIZATION

IgM antibodies are secreted as multisubunit polymers that consist of a s many as three discrete polypeptides: mu heavy chains, light (L) chai ns, and joining (J) chains, We wished to determine whether L chains th at are required to confer secretory competence on immunoglobulin molec ules must be present for IgM to polymerize-that is, for intersubunit d isulfide bonds to form between mu chains. Using a L-chain-loss variant of an IgM-secreting hybridoma, we demonstrated that mu chains were ef ficiently polymerized independent of L chains, in a manner similar to that observed for conventional mu L complexes, and that the mu polymer s incorporated J chain, These mu polymers were not secreted but remain ed associated with the endoplasmic reticulum-resident chaperone BiP (G RP78). This finding is consistent with the endoplasmic reticulum being the subcellular site of IgM polymerization. We conclude that mu chain alone has the potential to direct the polymerization of secreted IgM, a process necessary but not sufficient for IgM to attain secretory co mpetence.