ABSTRACT
The phosphorylation of proteins is an important and common mechanism for the intracellular regulation of enzymic activity. Numerous key regulatory enzymes are phosphorylated on specific residues at specific sites by the action of protein kinases in events that often alter the kinetic properties of the modified enzymes. Numbering among the protein-tyrosine kinases are the oncogene products of several acute transforming retroviruses and their cellular homologues, and the receptors for a variety of polypeptide growth factors. The best peptide substrates that have been described for protein-tyrosine kinases contain acidic amino acid residues near the phosphorylated tyrosine. The relative lack of specificity of protein-tyrosine kinases for small peptide substrates has led to the conjecture that secondary and tertiary structure play an important role in the recognition of protein substrates. Synthetic tyrosine-containing peptides can be phosphorylated in vitro by a wide variety of different protein-tyrosine kinases ranging from growth factor receptors to viral oncogene products.
