Catalytic domain of Tec-like Protein Tyrosine Kinases
PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. The Tec-like subfamily is composed of Tec, Btk, Bmx (Etk), Itk (Tsk, Emt), Rlk (Txk), and similar proteins. They are cytoplasmic (or nonreceptor) PTKs with similarity to Src kinases in that they contain Src homology protein interaction domains (SH3, SH2) N-terminal to the catalytic tyr kinase domain. Unlike Src kinases, most Tec subfamily members except Rlk also contain an N-terminal pleckstrin homology (PH) domain, which binds the products of PI3K and allows membrane recruitment and activation. In addition, some members contain the Tec homology (TH) domain, which contains proline-rich and zinc-binding regions. Tec kinases form the second largest subfamily of nonreceptor PTKs and are expressed mainly by haematopoietic cells, although Tec and Bmx are also found in endothelial cells. B-cells express Btk and Tec, while T-cells express Itk, Txk, and Tec. Collectively, Tec kinases are expressed in a variety of myeloid cells such as mast cells, platelets, macrophages, and dendritic cells. Each Tec kinase shows a distinct cell-type pattern of expression. Tec kinases play important roles in the development, differentiation, maturation, regulation, survival, and function of B-cells and T-cells. Mutations in Btk cause the severe B-cell immunodeficiency, X-linked agammaglobulinaemia (XLA). The Tec-like subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.