Identification of the SH2 domain binding protein of Bruton's tyrosine kinase as BLNK--functional significance of Btk-SH2 domain in B-cell antigen receptor-coupled calcium signaling.

Bruton's tyrosine kinase (Btk) is a critical component in the B-cell antigen receptor (BCR)-coupled signaling pathway. Its deficiency in B cells leads to loss or marked reduction in the BCR-induced calcium signaling. It is known that this BCR-induced calcium signaling depends on the activation of phospholipase Cgamma (PLCgamma), which is mediated by Btk and another tyrosine kinase Syk and that the SH2 and pleckstrin homology (PH) domains of Btk play important roles in this activation process. Although the importance of the PH domain of Btk has been explained by its role in the membrane targeting of Btk, the functional significance of the SH2 domain in the calcium signaling has remained merely a matter of speculation. In this report, we identify that one of the major Btk-SH2 domain-binding proteins in B cells is BLNK (B-cell linker protein) and present evidences that the interaction of BLNK and the SH2 domain of Btk contributes to the complete tyrosine phosphorylation of PLCgamma.