Answer to Question #340297 in Genetics for Dds

Question

Describe the role of cyclins, cyclin-dependent protein kinases, and protein degradation in the

progression of cells through the cell cycle

Answer 1

• Cyclins are proteins that regulate the cell cycle. There are 4 types of cyclins: cyclins G1, cyclins G1/S, cyclins S and cyclins M. Each of these cyclins takes part in the onset of a certain phase of the cell cycle in such a way that one or another phase begins with the beginning of the expression of this cyclin, or this phase begins only when the highest level of expression of this cyclin is reached.

As an example, below is a graph of the dependence of the concentration of various cyclins on the level of their expression at various phases of the cell cycle.

• Cyclins control the cell cycle by binding to cyclin-dependent protein kinases. By themselves, cyclin-dependent protein kinases are inactive, but they are activated only when cyclins are attached to them and they act as enzymes that phosphorylate and thereby activate target proteins of a certain phase of the cell cycle. For example, cyclin-dependent protein kinases, when M cyclins are attached to them, phosphorylate M-phase target proteins responsible for the destruction of the nuclear membrane.
• Protein degradation is mediated by the anaphase-promoting complex/cyclosome (APC/C), which is an enzyme complex that adds ubiquitin labels to target proteins in order to send them for degradation to the proteasome. This complex is needed in order to destroy M cyclins during the anaphase period, so that mitosis is completed and daughter cells enter the G1 phase. In addition, the APC/C complex also mediates the destruction of the cohesin that holds sister chromatids together, so that these chromatids separate and begin to move apart towards opposite poles of the cell. In fact, this complex first ubiquitinates securin, which is bound to the separase, thereby leaving the separase inactive. The ubiquitinated securin is then detached from the separase and sent for proteasomal degradation, whereby the separase becomes active and directly cleaves cohesin.