ABSTRACT
Phosphorylation of proteins is a major biological mechanism for regulating cellular function. Protein phosphorylation is involved in proliferation, growth and differentiation of all eukaryotic cells (for review see Krebs, 1993; Hunter, 1995). The various phosphorylation events are catalyzed by a large number of protein kinases, which phosphorylate their respective protein substrates at specific sites. The action of protein kinases can be reversed by protein phosphatases, which dephosphorylate phosphorylated proteins (for review see Cohen, 1992; Wera and Hemmings, 1995). The phosphorylation state of a protein is therefore determined by the counterplay of both, protein kinases and protein phosphatases, and phosphorylation/dephosphorylation is a powerful mechanism in the transduction and amplification of cellular signals.
