Multiple B29 containing complexes on murine B lymphocytes. Common and stage-restricted Ig-associated polypeptide chains.

The B29 gene is specifically expressed in all cells of the B lymphocyte lineage, and the B29 protein is disulfide-linked to the protein product of at least one other gene, known as mb-1. The noncovalent association of these heterodimers with Ig H chains is thought to be required for surface expression and signal transmission by B cell Ag receptors. We now demonstrate by two-color immunofluorescence a direct correlation between B29 density and surface Ig expression on normal spleen and bone marrow cells. The proportion of B29 in Ag receptor complexes appears to be relatively constant across major B lymphocyte subpopulations. Multiple B29-containing heterodimers were resolved on normal spleen cells by surface labeling, immunoprecipitation, two-dimensional gel analysis, and immunoblotting. As with lymphoma cells in our earlier study, the conditions of detergent extraction were critical to detection of certain species. Many laboratories have observed a family of 69- to 85-kDa heterodimers that are extracted with digitonin. These species are clearly Ig-associated, and are coprecipitated with anti-Ig antibodies. We found that extraction with Triton X-100 revealed an additional pair of 52- to 58-kDa heterodimers, where B29 was disulfide-bonded to a protein of approximately 23 kDa. The latter was detectable by immunoblotting with antibodies to extracellular, but not cytoplasmic, portions of mb-1. We found that, with mature cells, both conventional and low molecular mass heterodimers were solubilized with digitonin, but only detectable if Triton was present during immunoprecipitation. Thus, a protein having partial serologic identity with mb-1 forms heterodimers that are cryptic on splenic B cells, and possibly not directly associated with surface Ig molecules. In contrast, both types of heterodimers were readily detectable on late stage pre-B cells, regardless of detergent used for extraction or antibody used for immunoprecipitation. In that situation, both low- and high molecular mass heterodimers were associated with surface Ig. These findings increase our understanding of the B lymphocyte Ag receptor complex and indicate that its components may change as a function of differentiation.