| Research Abstract |
We are investigating the molecular pathogenesis of antibody deficiencies, especially X-linked agammaglobulinemia (XLA) caused by mutations in Bruton's tyrosine kinase (Btk). Several studies have indicated that Btk is responsible for the B cell receptor-coupled calcium signaling. We previously reported that Btk and another tyrosine kinase Syk coordinately contributes to the activation of phospholipase Cγ2 (PLCγ2) upon the adaptor molecule BLNK. As the next step to clarify the Btk-dependent signaling in B cells, we investigated the exact molecular mechanism whereby Btk activates PLCγ2. We identify the tyrosine residues 753, 759, 1197, and 1217 in PLCγ2 as Btk-dependent phosphorylation sites. To evaluate the role of these tyrosine residues in phosphorylation-dependent activation of PLCγ2, PLCγ2-deficient B cells were reconstituted with a series of mutant PLCγ2 in which the phenylalanine was substituted for tyrosine. Cells expressing PLCγ2 with a single substitution exhibited some extent of reduction in calcium mobilization, whereas those expressing quadruple mutant PLCγ2 showed greatly reduced calcium response. These findings indicate that the phosphorylations of the tyrosine residues 753, 759, 1197, and 1217, which have been identified as Btk-dependent phosphorylation sites in vitro, coordinately contribute to BCR-induced activation of PLCγ2.
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