Heuristic models of the Intermonomeric Disulfide Bonding Process

The assembly of fully disulfide bonded trout Immunoglobulin M (IgM) was modeled heuristically. The basic mechanism was assumed to be the sequential formation of single disulfide bonds (DSB), linking two monomeric subunits. Biochemical details of the mechanisms of bond formation were not considered. The progression of bonding within each tetramer was described entirely in terms of the rates of bond formation. It was further assumed that the sequence of DSB formation from non-covalently associated tetramers to fully bonded tetramers progressed by the formation of one bond at a time. The experimentally observed ratios of covalently-linked subunits within the tetramers were compared with the values predicted by the models. The results of these analyses suggest that the final stages of trout Ig assembly (the DSB formation between monomeric subunits) must occur in intracellular compartments late in the secretory process itself, thereby yielding incompletely crosslinked tetramers. Further, it would appear that the rate of disulfide bond formation within a tetramer may be accelerated by the presence in that tetramer, of previously incurred, intermonomeric DSBs.