Control of glycogen synthase by hierarchal protein phosphorylation.

Protein phosphorylation is one of the most common mechanisms for controlling protein function. We now know that most phosphoproteins contain multiple phosphorylation sites and that these sites are often located in clusters. From the study of the enzyme glycogen synthase, one mechanism for the formation of phosphorylation clusters has been discovered that involves the concerted action of two or more protein kinases. One protein kinase, the primary kinase, introduces a phosphate group that is a requirement for the action of another, secondary, protein kinase. Thus the multiple phosphorylation occurs in a hierarchal fashion. This mechanism, which is critical for the phosphorylation of glycogen synthase, is likely to be a much more widespread phenomenon.