Molecular components of the B-cell antigen receptor complex of the IgM class.

The antigen receptors on mature B lymphocytes are membrane-bound immunoglobulins of the IgM and IgD classes whose cross-linking by polyvalent antigens results in B-cell proliferation and differentiation. How these membrane-bound immunoglobulin chains, which lack a cytoplasmic tail, generate a cell activation signal is not at present known. We now show that the IgM molecule is non-covalently associated in the membrane of B cells with two proteins of relative molecular mass 34,000 (Mr 34 K; IgM-alpha) and 39 K (Ig-beta) which form a disulphide-linked heterodimer. Surface expression of IgM seems to require the formation of an appropriate complex between IgM and the heterodimer. A transfection experiment indicates that IgM-alpha is the product of mb-1, a B-cell specific gene encoding a transmembrane protein with sequence homology to proteins of the T-cell antigen receptor-CD3 complex.