EVIDENCE FOR A DIRECT PHYSICAL INTERACTION OF MEMBRANE IGM, IGD, AND IGG WITH THE B29 GENE-PRODUCT

The B cell Ag-receptor complex is composed of membrane immunoglobulin (mlg) and the mb-1/B29 heterodimer. In order to obtain insight into th e architecture of the B cell receptor complex, we have looked for cond itions that disrupt all disulfide bridges in the complex without affec ting the noncovalent interaction between the mig heavy chain and one o r both members of the associated heterodimer. We show that in the pres ence of the reducing agent beta-mercaptoethanol the m mu, m delta, and m gamma heavy chains remain selectively associated with the B29 membe rs. Our findings implied that if isotype-related differences exist bet ween the mlg-associated dimers, they may reside in B29 and not, as ini tially suggested, in mb-1. However, sequence analyses of B29 gene tran scripts from B cells expressing mlgM, mlgD, or mlgG only revealed no d ifferences in their nucleotide composition. Thus, in spite of their cl ose physical interaction with mig heavy chain classes, which are signi ficantly distinct in the C-terminal regions, no isotype-specific forms of B29 seem to exist.