Protein Tyrosine Kinase (PTK) is an important enzyme of the signal transduction pathway which controls cell proliferation, differentiation and carcinogenesis. PTK is classified into two groups: 1) membrane receptor PTKs (e.g. insulin receptor and EGF receptor) and 2) non-receptor linked PTKs (e.g. Src and FAK). Receptor PTKs contain three domains: an extracellular domain, a membrane intrinsic domain, and an intracellular domain. The extracellular and membrane intrinsic domains bind to ligands such as hormones and growth factors, while the intracellular domain carries the PTK active site. Receptor PTKs transduce signals directly into the cell by phosphorylation of tyrosine when ligands bind to their domains. Non-receptor linked PTKs lack the membrane intrinsic domain and are divided into three types based upon their location in the cell (i.e. membranes, cytoplasm, or nucleus). Non-receptor linked PTKs participate in various functions, including transduction and cell-cell contact responses.
