The reversible phosphorylation of proteins is a key mechanism whereby signalling cascades involved in the response to extracellular stimuli bring about changes in cellular function. These proteins include the kinases/phosphatases that form such signaling pathways as well as the transcription factors involved in inducible changes in gene expression (1 ). Phosphorylation induces changes in the function of these proteins either by induction of allosteric conformational changes in the protein itself or in the regulation of its interaction with other cellular factors.
