The cell cycle is driven by a class of protein kinases called cyclin-dependent kinases (CDKs), which by definition, are activated by binding to cyclins. Cyclins were first identified as proteins that oscillate in the cell cycle. Cyclin A is important for S phase and is degraded rapidly during G₂, slightly prior to the destruction of cyclin B. The ubiquitin-mediated proteolysis pathway is implicated in the control of cyclin destruction. Cyclin B degradation mechanisms by anaphase-promoting complex are well studied, but the destruction of cyclin A remains obscure. In one study, I measured the levels of cyclins and their CDK partners in HeLa cells using a novel technique. I found that the peak levels of cyclin A and cyclin B are about 30-fold less than the level of CDC2 (also known as CDK...[ Read more ]

The cell cycle is driven by a class of protein kinases called cyclin-dependent kinases (CDKs), which by definition, are activated by binding to cyclins. Cyclins were first identified as proteins that oscillate in the cell cycle. Cyclin A is important for S phase and is degraded rapidly during G₂, slightly prior to the destruction of cyclin B. The ubiquitin-mediated proteolysis pathway is implicated in the control of cyclin destruction. Cyclin B degradation mechanisms by anaphase-promoting complex are well studied, but the destruction of cyclin A remains obscure. In one study, I measured the levels of cyclins and their CDK partners in HeLa cells using a novel technique. I found that the peak levels of cyclin A and cyclin B are about 30-fold less than the level of CDC2 (also known as CDKl); but the peak levels of cyclin A and cyclin E are only about 8-fold less than the level of CDK2. These results confirmed the long-held presumption that CDKs are in excess of the cyclins in the cell. In another study, I characterized a novel cyclin A binding protein SKP2. SKP2 could inhibit cyclin A-CDK2 kinase by both direct binding and by blocking the activating phosphorylation by the CDK-activating kinase (CAK). Overexpression of SKP2 arrested cells in G₁/S, probably through its inhibition of CDK2. To study the regulation of cyclin A by phosphorylation, I have identified the CDK phosphorylation site of cyclin A as Ser-154. However, ubiquitination and degradation of cyclin A, and binding of cyclin A to its interacting proteins were not affected by Ser-154 phosphorylation. It is likely that cyclin A degradation is not regulated by direct CDK2 phosphorylation. Finally, I disproved the result that cyclin A is cleaved at R70-R71 by a p27^KIP1-activated protease described in previous studies.