Chain-Deficient IgM Structural and Functional Analysis of J

some arrangement of inter-␮ bonds directs incorporation of J chain. IgM-S337 hexamer also dissociated to ␮2␬2 and ␮4␬4, but also yielded several species migrating much more slowly in SDS-PAGE than wild-type ␮12␬12. To account for these forms, we propose that each ␮-chain can interact with three other ␮-chains and that some hexameric molecules contain two catenated ␮6␬6 circles. J chain is a 15-kDa glycoprotein covalently associated by di-sulfide bonds with polymeric forms of IgM and IgA (1, 2). The Ig ␮ heavy chain has three cysteine residues available for intermolecular disulfide bonds. C337, C414, and C575 lie in the C␮2, C␮3, and C␮4/tail domains, respectively, and are thought to join ␮-chains via homo-pairs (C337-C337, C414-C414, and C575-C575) (3, 4). Each pentameric IgM molecule contains approximately one J chain attached to the penultimate amino acid of the ␮-chain, C575 (2, 5– 8). J chain is disulfide bonded via its C14 and C68 to no more than two ␮-chains (6, 9). Most or all incorporation of J chain into IgM occurs relatively late in the intracel-lular assembly process, at a stage when IgM polymers have been formed but are not yet fully linked by disulfide bonds (5, 10). Two functions have been described for Ig-associated J chain. J chain is necessary for binding of IgM and IgA to the poly-Ig receptor , which functions in epithelial transcytosis of Igs (11–13). Also, J chain modifies polymeric assembly of IgM, in that relatively more pentamer and less hexamer are produced in the presence of J chain. The fraction of IgM that was seen as pentamer in the absence of J chain has varied among different studies, ranging from none up to 50% (14 –16). Pentameric IgM differs from hex-americ IgM in several ways. First, J chain is found in pentameric, but not hexameric, IgM. Second, pentameric IgM is less cytolytic than hexameric IgM (17, 18). Third, analysis of mutant IgM has suggested that more inter-␮-chain C414-C414 bonds are formed in hexameric IgM than in pentameric IgM (19, 20). These correlations could have multiple explanations. For example, incorporation of J chain into pentamer might affect the disulfide bonding of IgM and in this indirect way depress its cytolytic activity. Alternatively, the hexameric C1q complement component might interact better with hexameric than with pentameric IgM. As well, IgM-bound J chain might sterically hinder the binding of C1q and thereby decrease the cytolytic activity of pentameric IgM. …