Protein tyrosine kinases, with emphasis on the Src family

Protein tyrosine phosphorylation is recognized to be an important regulatory event, particularly in those pathways that control cell growth and differentiation. Phosphorylation on tyrosine may either decrease or increase the enzymatic activity of substrate proteins. In addition, tyrosine phosphorylated sequences associate with Src homology 2 (SH2) domains, and thus tyrosine phosphorylation also serves to regulate protein/protein interactions. Many protein tyrosine kinases have been described to date: several are the receptors for peptide growth factors; others are expressed in the cytoplasm and nucleus. The main subject of this review will be the Src family of protein tyrosine kinases, a group of nine enzymes thought to play important roles in several signal transduction pathways.