Post-translational phosphorylation is one of the most common protein modifications that occur in animal cells. The vast majority of phosphorylation occurs as a mechanism of acute and reversible regulation of protein function. Studies of mammalian cells metabolically labeled with p32 orthophosphate suggest that as many as one-third of all cellular proteins are covalently modified by protein phosphorylation. Covalent attachment of a phosphate group to an amino acid side chain of a protein can cause a structural change, for example, by attracting a cluster of positively charged side chains. Such...