PHOSPHORYLATED IMMUNORECEPTOR SIGNALING MOTIFS (ITAMS) EXHIBIT UNIQUEABILITIES TO BIND AND ACTIVATE LYN AND SYK TYROSINE KINASES

Signal transduction by T and B cell Ag receptors and certain receptors for Ig Fc regions (Fc gamma RI, hFc gamma RIIA, Fc gamma RIII, Fc alp ha R, and Fc epsilon RI) involves a conserved sequence motif, termed a n immunoreceptor tyrosine-based activation motif (ITAM) and found in m ultiple receptor chains. Phosphorylation of the two ITAM tyrosines is a critical event in signal transduction. To address the function of th is phosphorylation, we assessed the ability of nonphosphorylated and b iphosphorylated ((p)(2)ITAM) ITAM peptides to bind and modify the acti vity of src and syk family kinases in vivo and in vitro. All (p)(2)ITA Ms, but not their nonphosphorylated counterparts, induced extensive pr otein tyrosine phosphorylation in permeabilized cells. However, the pa tterns of proteins phosphorylated differed among (p)(2)ITAMs. This pho sphorylation was found to reflect activation of the src family kinase Lyn, but not Syk. In vitro studies using purified Lyn showed that src family kinase activation resulted from a direct interaction with (p)(2 )ITAM. Binding studies demonstrated clear differences in binding speci ficity of (p)(2)ITAMs. Most strikingly, Ig alpha (p)(2)ITAM and TCR-ze ta c and CD3 epsilon (p)(2)ITAMs exhibit inverse binding preferences f or src and syk family kinases. Taken together, these findings demonstr ate a novel mechanism by which src family tyrosine kinases are activat ed, and are consistent with the possibility that different ITAMs may p referentially activate distinct signaling pathways as a consequence of distinct effector Src homology 2 domain (SH2) binding preference.