TI - Tec kinases mediate sustained calcium influx via site -specific tyrosine phosphorylation of the phospholipase Cgamma Src homology 2-Src homology 3 linker . AB - Tyrosine phosphorylation of phospholipase Cgamma2 ( PLCgamma2 ) is a crucial activation switch that initiates and maintains intracellular calcium mobilization in response to B cell antigen receptor ( BCR ) engagement . Although members from three distinct families of non - receptor tyrosine kinases can phosphorylate PLCgamma in vitro , the specific kinases controlling BCR -dependent PLCgamma activation in vivo remains unknown . Bruton's tyrosine kinase ( Btk ) -deficient human B cells exhibit diminished inositol 1, 4, 5-trisphosphate production and calcium signaling despite a normal inducible level of total PLCgamma2 tyrosine phosphorylation . This suggested that Btk might modify a critical subset of residues essential for PLCgamma2 activity . To evaluate this hypothesis , we generated site -specific phosphotyrosine antibodies recognizing four putative regulatory residues within PLCgamma2 . Whereas all four sites were rapidly modified in response to BCR engagement in normal B cells , Btk-deficient B cells exhibited a marked reduction in phosphorylation of the Src homology 2 ( SH2 ) -SH3 linker region sites , Tyr(753) and Tyr(759) . Phosphorylation of both sites was restored by expression of Tec , but not Syk , family kinases . In contrast , phosphorylation of the PLCgamma2 carboxyl - terminal sites , Tyr(1197) and Tyr(1217) , was unaffected by the absence of functional Btk . Together , these data support a model whereby Btk/Tec kinases control sustained calcium signaling via site -specific phosphorylation of key residues within the PLCgamma2 SH2-SH3 linker .