FIGURE 15-3 Regulation of protein activity by a kinase/phosphatase switch. The cyclic phosphorylation and de-phosphorylation of a protein is a common cellular mechanism for regulating protein activity. In this example, the target, or substrate, protein is inactive (light green) when not phosphorylated and active (dark green) when phosphorylated; some proteins have the opposite pattern. Both the protein kinase and the phosphatase act only on specific amino acids in specific target proteins, and their activities are usually highly regulated.