Non-receptor protein tyrosine kinases.

The protein tyrosine kinases (PTKs) are enzymes catalyzing the transfer of the gamma-phosphate group of ATP to the hydroxyl groups of specific tyrosine residues in peptides. Although phosphotransfer reactions catalyzed by various PTKs are similar with regard to their basic mechanisms, their biological functions demonstrate a considerable degree of specificity. PTKs are divided into two groups according to the presence of transmembrane and extracellular domains. Whereas most PTKs possess these domains, which render them capable of recognizing extracellular ligands, many PTKs lack these sequences and are therefore referred to as non-receptor or non-transmembrane PTKs. Thirty-two genes encoding for non-receptor PTKs are present in the human genome. The current review focuses on the composition, structure, expression, functions and regulation of the mammalian non-receptor PTK families.