LATE EVENTS IN ASSEMBLY DETERMINE THE POLYMERIC STRUCTURE AND BIOLOGICAL-ACTIVITY OF SECRETORY IGM

IgM antibodies can be secreted in at least two functional polymeric fo rms that can be distinguished according to subunit composition. While IgM hexamers comprise six H2L2 monomeric subunits, pentamers contain a n additional polypeptide, the J chain. In the presence of high abundan ce J chain protein, IgM pentamers are preferentially assembled at the expense of hexamers. To determine the mechanism by which J chain regul ates the assembly process, we defined the point at which J chain is ad ded to assembling polymers. We found no evidence for the presence of J chain in small IgM assembly intermediates of IgM, suggesting that it was not stably associated with these complexes. However, J chain was f ound associated with large polymeric IgM complexes exhibiting sediment ation properties of intracellular pentameric structures. These complex es were frequently not completely covalently assembled; however, compl ete covalent assembly of J chain-containing pentameric complexes did o ccur prior to their maturation in the Golgi. These data argue that pen tameric structures are the substrate for J chain incorporation into as sembling IgM and suggest that the incorporation of J chain is thermody namically favored over the addition of a sixth monomeric subunit into an assembling polymer. We conclude that late events in IgM polymer ass embly, specifically the insertion of J chain, the exclusion of an addi tional monomeric subunit, and the covalent closure of the pentameric I gM molecule, determine the polymeric structure and, consequently, the biological activity of secreted IgM. (C) 1997 Elsevier Science Ltd.