ABSTRACT
Covalent reversible phosphorylation of cellular proteins is an important regulatory mechanism involved in the control of metabolism as well as in the regulation of cell proliferation and differentiation. 1-3 The phosphorylations regulated by protein kinases occur most frequently on serine/threonine residues, but phosphorylation on tyrosine, although less frequent, is very important for the functional modification of cellular proteins. Some of the phosphorylated cellular proteins are located in the nucleus and are directly involved in the control of RNA synthesis and DNA replication. Nuclear proteins whose functions are regulated by phosphorylation at specific amino acid residues include proto-oncogene products such as Myc, Myb, Fos, Jun, and Ets, as well as the products of tumor suppressor genes such as the p53 and RB proteins. Transcription factors such as CREB and NF-κB are substrates for kinases involved in the control of cell growth.
