Receptor Tyrosine Kinases ( RTKs) activate numerous signaling pathways within cells, leading to cell proliferation, differentiation, migration, or metabolic changes. In addition to the RTKs, there exists a large family of NonReceptor Tyrosine Kinases ( NRTKs), which includes Src, the Janus kinases ( Jaks), and Abl, among others. The NRTKs are integral components of the signaling cascades triggered by RTKs and by other cell surface receptors such as G protein-coupled receptors and receptors of the immune system. The specific reaction catalyzed by Protein Tyrosine Kinases ( PTKs) is the transfer of the phosphate of ATP to the hydroxyl group of a tyrosine in a protein substrate.
The largest subfamily of NRTKs, with nine members, is the Src family. Src family members participate in a variety of signaling processes, including mitogenesis, T- and B-cell activation, and cytoskeleton restructuring.
Signaling by activated T and B cells of the immune system is dependent on multiple NRTKs. For instance, upon stimulation of the B-cell receptor, the Src family member Lyn is activated, which leads to the recruitment and phosphorylation of Syk, a Zap70-related NRTK. Another NRTK, Btk, also plays an important role in B-cell signaling. Mutations in the Btk gene are responsible for X-linked agammaglobulinemia, a disease characterized by the lack of mature B cells.
(Protein Tyrosine Kinase Structure And Function, 2000)