Cooperativity and segregation of function within the Ig-alpha/beta heterodimer of the B cell antigen receptor complex.

The B cell antigen receptor complex contains heterodimers of Ig-alpha and Ig-beta. The cytoplasmic tails of each of these chains contain two conserved tyrosines, phosphorylation of which initiates the signal transduction cascades activated by the receptor complex. Although the cytoplasmic domains of Ig-alpha and Ig-beta have been expressed individually and demonstrated to be competent signal transduction units, we postulated that within the context of a heterodimer, Ig-alpha and Ig-beta could have new, complementary or even synergistic functions. Therefore we developed a system to compare the signal transducing capacities of dimers of Ig-alpha/Ig-alpha, Ig-beta/Ig-beta, or Ig-alpha/Ig-beta. This was done by fusing the extracellular and transmembrane domains of either human platelet-derived growth factor receptor (PDGFR) alpha or beta to the cytoplasmic tail of either Ig-alpha or Ig-beta. Three cell lines expressing PDGFRbeta/Ig-alpha, PDGFRbeta/Ig-beta, or PDGFRalpha/Ig-beta together with PDGFRbeta/Ig-alpha were established in the murine B cell line A20 IIA1.6. While aggregation of each dimer by itself could induce the tyrosine phosphorylation of cellular substrates, only aggregation of the heterodimer induced the phosphorylation of substrates similar in range and intensity to that induced by the endogenous B cell antigen receptor complex. Interestingly, Ig-beta remarkably enhanced the rapidity (Tmax decreased from 5 to 1 min) and intensity (greater than 10-fold enhancement) of Ig-alpha phosphorylation. Conversely, the phosphorylation of Ig-beta was reduced to undetectable levels when co-aggregated with Ig-alpha. The enhancement of Ig-alpha phosphorylation by Ig-beta correlated with a lowering of the stimulation threshold for tyrosine kinase activation.