Influence of the mu-chain C-terminal sequence on polymerization of immunoglobulin M

Immunoglobulin (IgM) is found in various states of covalent polymerization (mu L)(n), where n is typically 8, 10, or 12. The usual form of IgM of bony fish is tetrameric (8 mu L units) as compared to the pentameric form (10 m u L units) observed in cartilaginous fish and mammals. Two hypotheses were tested in this study. First, that the length of the mu-chain C terminus fol lowing Cys575 determines whether an IgM polymerizes as a tetramer or as a p entamer. This was tested by examining the covalent polymerization state of mouse IgM mutated to contain a series of mu-chain C-termini from bony and c artilaginous fish. The results proved this hypothesis wrong: mouse IgM bear ing the C-terminal sequence of shark, salmon and cod mu-chain behaved ident ically to native mouse IgM, forming predominantly (mu L)(10) and (mu L)(12) forms. The second hypothesis was that an additional Cys residue near the C terminus of the mu-chain is responsible for the multiple covalent structur es seen in IgM of the channel catfish. The addition of a catfish C terminus to the mouse mu-chain resulted, as predicted, in the production of a serie s of covalently bonded forms, with the major species being (mu L)(4). When a Ser-Cys unit was removed from the catfish C terminus added to the mouse m u-chain, this resulted in production of IgM indistinguishable in structure from that of wild-type mouse IgM.