ASYMMETRICAL PHOSPHORYLATION AND FUNCTION OF IMMUNORECEPTOR TYROSINE-BASED ACTIVATION MOTIF TYROSINES IN B-CELL ANTIGEN RECEPTOR SIGNAL-TRANSDUCTION

CD79a and CD79b function as transducers of B cell antigen receptor sig nals via a cytoplasmic sequence, termed the immunoreceptor tyrosine-ba sed activation motif (ITAM), ITAMs contain two conserved tyrosines tha t may become phosphorylated upon receptor aggregation and bind distinc t effecters by virtue of the distinct preference of phosphotyrosyl-con taining sequences for SH2 domains. To explore the function of CD79a an d CD79b ITAM tyrosines, we created membrane molecules composed of MHC class II I-A(k) extracellular and transmembrane domains, and CD79a or CD79b cytoplasmic domains in which one or both of the ITAM tyrosines w ere mutated to phenylalanine. Functional analysis revealed that both I TAM tyrosines are required for ligand-induced Syk phosphorylation, How ever CD79a-ITAM and CD79b-ITAM tyrosine phosphorylations were asymmetr ical, with >80% of phosphorylation occurring on the N-terminal tyrosin e (Y-E-G-L), Thus, these findings suggest that following receptor liga tion, only a minor proportion of phosphorylated ITAMs are doubly phosp horylated and thus can engage Syk, Only the N-terminal ITAM tyrosine o f CD79a was required for ligand-mediated phosphorylation of the recept or and a subset of downstream substrates, including p62, p110, and Shc , and for Ca2+ mobilization, However, responses mediated through CD79b exhibited a greater dependence on the presence of both tyrosines, Nei ther tyrosine in CD79a or CD79b appeared absolutely essential for Src family kinase phosphorylation. These results indicate that phosphoryla tions of the tyrosines in CD79a and CD79b occur with very different st oichiometry, and the respective tyrosyl residues have distinct functio ns.