Tyrosine Residues in Phospholipase Cγ2 Essential for the Enzyme Function in B-cell Signaling

Phospholipase Cγ (PLCγ) isoforms are regulated through activation of tyrosine kinase-linked receptors. The importance of growth factor-stimulated phosphorylation of specific tyrosine residues has been documented for PLCγ1; however, despite the critical importance of PLCγ2 in B-cell signal transduction, neither the tyrosine kinase(s) that directly phosphorylate PLCγ2 nor the sites in PLCγ2 that become phosphorylated after stimulation are known. By measuring the ability of human PLCγ2 to restore calcium responses to the B-cell receptor stimulation or oxidative stress in a B-cell line (DT40) deficient in PLCγ2, we have demonstrated that two tyrosine residues, Tyr⁷⁵³ and Tyr ⁷⁵⁹, were important for the PLCγ2 signaling function. Furthermore, the double mutation Y753F/Y759F in PLCγ2 resulted in a loss of tyrosine phosphorylation in stimulated DT40 cells. Of the two kinases that previously have been proposed to phosphorylate PLCγ2, Btk, and Syk, purified Btk had much greater ability to phosphorylate recombinant PLCγ2 in vitro, whereas Syk efficiently phosphorylated adapter protein BLNK. Using purified proteins to analyze the formation of complexes, we suggest that function of Syk is to phosphorylate BLNK, providing binding sites for PLCγ2. Further analysis of PLCγ2 tyrosine residues phosphorylated by Btk and several kinases from the Src family has suggested multiple sites of phosphorylation and, in the context of a peptide incorporating residues Tyr ⁷⁵³ and Tyr⁷⁵⁹, shown preferential phosphorylation of Tyr⁷⁵³.