B29 gene products complex with immunoglobulins on B lymphocytes.

B29 is a B-lineage-specific gene predicted from sequence information to be a transmembrane member of the immunoglobulin (Ig) superfamily, with a single extracellular Ig-like domain. Its presumptive cytoplasmic region contains a peptide motif present in CD3 and other molecules involved in lymphocyte activation. Affinity-purified goat antibodies were prepared to a TrpE fusion protein of B29 and used to study B29 expression on lymphoid cells. The antiserum precipitated surface-labeled heterodimers from B lymphoma cells. One was 65-88 kDa (unreduced) or 36-47 plus 32-34 kDa (reduced) by SDS/PAGE analysis, regardless of detergent. A smaller heterodimer was detected only with Triton detergent extraction. IgM molecules were coprecipitated by the B29 antiserum when the weak detergent digitonin was used. In addition, cocapping experiments revealed that most B29 molecules codistribute with Ig on the cell surface. Although early B-lineage cells and plasma cells contain B29 mRNA, surface expression was detectable only on B cells that had significant amounts of surface Ig. The surface expression was B-lineage-specific and included cells from mutant xid mice and B-cell lines representing mu, delta, gamma, and alpha heavy-chain isotypes and both kappa and lambda light-chain types. The density of surface B29 protein correlated directly with surface mu heavy-chain density on subclones of a B-cell lymphoma and lipopolysaccharide-stimulated pre-B cells. These findings show that B29 is covalently linked in a heterodimer and are consistent with a recently proposed model of surface Ig complexes.