IgM antigen receptor complex contains phosphoprotein products of B29 and mb-1 genes

Membrane immunoglobulin M (mIgM) and mIgD are major B-lymphocyte antigen receptors, which function by internalizing antigens for processing and presentation to T cells and by transducing essential signals for proliferation and differentiation. Although ligation of mIgM or mIgD results in rapid activation of a phospholipase C and a tyrosine kinase(s), these receptors have cytoplasmic tails of only three amino acid residues (Lys-Val-Lys), which seem ill suited for direct physical coupling with cytoplasmic signal transduction structures. In this report, we identify the α, β, and γ components of the mIgM-associated phosphoprotein complex, which may play a role in signal transduction. Proteolytic peptide mapping demonstrated that the IgM-α chain differs from Ig-β and Ig-γ. The chains were purified, and amino-terminal sequencing revealed identity with two previously cloned B-cell-specific genes. One component, IgM-α, is a product of the mb-1 gene, and the two additional components, Ig-β and Ig-γ, are products of the B29 gene. Immunoblotting analysis using rabbit antibodies prepared against predicted peptide sequences of each gene product confirmed the identification of these mIgM-associated proteins. The deduced sequence indicates that these receptor subunits lack inherent protein kinase domains but include common tyrosine-containing sequence motifs, which are likely sites of induced tyrosine phosphorylation.