Proteins That Regulate the Cycle

The mammalian cell cycle control system is regulated by a group of protein kinases called cyclin-dependent kinases (CDKs). These proteins catalyze the attachment of phosphate groups to specific serine or threonine amino acids in a target protein. The phosphate groups alter the target protein's properties, such as its interaction with other proteins. (The alteration of protein activity by the attachment of phosphate groups occurs frequently in cells.)

CDKs are called "cyclin-dependent" because their activity requires their association with activating subunits called cyclins. While the number of CDKs in a cell remains constant during the cell cycle, the levels of cyclins apoptosis programmed cell death kinases enzymes that add a phosphate group to another molecule, usually a protein phosphate groups

PO43~ groups, whose presence or absence often regulates protein action

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