The structure of the μ/pseudo light chain complex on human pre‐B cells is consistent with a function in signal transduction

Prior to immunoglobulin (Ig) light (L) chain rearrangement, pre‐B cells can express μ heavy (H) chains at the cell surface in association with pseudo (ψ) L chains. This complex may be essential for B cell development. We have investigated the composition of the μ/ψL chain complex of a human pre‐B cell line, in view of its potential role in transmembrane signal transduction. The μ/λ. receptor of a mature B cell line was analyzed in comparison. The μ/ψL chain complex is associated with disulfide‐linked molecules that are homologous or identical to the mb‐1 and B29 proteins, known to be integral components of membrane Ig receptors on mature B cells. Both receptors contain tyrosine (Tyr) kinase activity. In the μ/λ receptor, the lyn and lck Tyr kinases could clearly be identified. The mb‐1 and B29 proteins in both μ/λ and μ/ψL chain receptors are substrates for in vitro phosphorylation on Tyr, but also on serine (Ser) and threonine (Thr) residues. The undefined μ‐associated Ser/Thr kinase also phosphorylates the sre‐related kinases in the μ/λ, receptor and a 43‐kDa μ‐associated protein that is present in both complexes. The 43‐kDa protein may be an integral part of both receptor types, or a transiently associated molecule instrumental in the signaling process. We conclude that the μ/ψL receptor on human pre‐B cells fulfills the presently known criteria to function as a signal transduction unit.