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

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Abstract
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.