On the importance of protein phosphorylation in cell cycle control

This chapter is written as a contribution to a volume commemorating the work of Krebs and Fischer that led to awarding of the Nobel Prize in 1992. This award was made because of their fundamental discovery in the mid-1950s that protein phosphorylation was the underlying mechanism that accounted for the reversible modification of activity of glycogen phosphorylase in mammalian skeletal muscle. Although it could not be anticipated at the time that phosphorylation would turn out to be such a ubiquitous regulator of cellular functions, it is now evident that phosphorylation controls virtually every important reaction in cells and provides the basis for understanding how integrated cellular behavior is regulated by both extracellular signals and internal control mechanisms. This chapter relates the historical development in biochemical terms of protein phosphorylation as a regulator of the cell cycle in Xenopus oocytes and eggs.