Résumé
Phosphorylation on tyrosine residues accounts for only 0.05% of total protein phosphorylation in mammalian cells, yet protein tyrosine kinases play important roles in the intracellular transduction of extracellular signals. They are involved in various biological responses including cell growth and differentiation, cell motility, and cell secretion. Protein tyrosine kinases can be divided into two groups: receptor and non-receptor tyrosine kinases. The receptor class comprises a family of cell surface proteins, which act as receptors for a variety of ligands. They are composed of an extracellular domain which contains the ligand binding site, a transmembrane domain, and a cytoplasmic domain that contains catalytic activity. This class of kinase is detailed in Chapter 3. Members of the second class of tyrosine kinases do not have extracellular sequences and do not span the plasma membrane. They can be located in the cytoplasm, on the inner part of the plasma membrane, or in the nucleus. The cytoplasmic tyrosine kinases have been sub classified into a number of families based upon their structural homology.