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.

Varied redox forms of teleost IgM: an alternative to isotypic diversity?

Teleosts (bony fish) are thought to primarily or exclusively possess a single structural form of immunoglobulin (Ig), a tetrameric IgM. However, in species wherein intact Ig has been electrophoretically analyzed under denaturing, non-reducing conditions, a significant degree of structural diversity has been revealed. This IgM molecule appears to be assembled with great latitude in the degree of disulfide crosslinking between monomeric or halfmer subunits composing the complete IgM molecule. This heterogeneity in the basic structure (herein referred to as redox forms) is not due to isotypic differences as each B cell produces this heterogeneity within its immunoglobulin product. Additionally, in the case of the catfish, a single fish/mouse chimeric Ig H gene is capable of producing IgM with a comparable amount of structural heterogeneity within the mouse cell. Thus, the piscine B lymphocyte routinely assembles a variety of redox forms from one IgM chain. This has both profound biosynthetic implications for macromolecular assembly processes as well as intriguing possibilities for the generation of teleost Ig functional diversity.