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

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