THE STRUCTURE OF THE MU 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 cell s ca express mu heavy (H) chains at the cell surface in association wi th pseudo (psi) L chains. This complex may be essential for B cell dev elopment. We hav investigated the composition of the mu/psiL chain com plex of a human pre-B cel line, in view of its potential role in trans membrane signal transduction. The mu/lambda receptor of a mature B cel l line was analyzed in comparison. The mu/psiL chain complex is associ ated with disulfide-linked molecules that are homologous or identical to the mb-1 and B29 proteins, known to be integral components of membr ane Ig receptors on mature B cells. Both receptors contain tyrosine (T yr kinase activity. In the mu/lambda receptor, the lyn and Ick Tyr kin ases could clearly b identified. The mb-1 and B29 proteins in both mu/ lambda and mu/psiL chain receptors ar substrates for in vitro phosphor ylation on Tyr, but also on serine (Ser) an threonine (Thr) residues. The undefined mu-associated Ser/Thr kinase als phosphorylates the src- related kinases in the mu/lambda receptor and a 43-kD mu-associated pr otein that is present in both complexes. The 43-kDa protein ma be an i ntegral part of both receptor types, or a transiently associated molec ule instrumental in the signaling process. We conclude that the mu/psi L receptor on human pre-B cells fulfills the presently known criteria to function as a signal transduction unit.