Cyclin Proteolytic degradation

The biochemical mechanism of Mos action is not yet established. Mos has been found to phosphorylate cyclin B in vitro, and it is possible that this phosphorylation directly inhibits cyclin B proteolysis (Roy et al., 1990). However, such a direct effect of phosphorylation on cyclin B stability remains to be demonstrated, and it is alternatively possible that Mos inhibits (directly or indirectly) the proteolytic pathway responsible for cyclin B degradation. Mos has recently been found to stimulate mitogen-activated protein kinase (MAP kinase) in Xenopus oocytes,... 

Finally, the cyclin is degraded by a proteolytic enzyme so that the cyclin concentration decreases to zero and the kinase is now completely inactive and cannot be activated again until the cyclin is, once again, synthesised and binds to the naked kinase, i.e. degradation of the cyclin arrests the cycle. 

Increase in cyclin concentration by activation of banscription or inhibition of proteolytic degradation... 

The cell cycle The concentrations of specific proteins regulate the cell cycle by activation (known as cyclins) is achieved of cell-division cycle kinases, during key steps in the cycle. The concentration of these proteins (cyclins) is regulated by synthesis and degradation. The latter is this proteolytic system (Chapter 20). 

The progression of the cell cycle is regulated by interconversion processes, in each phase, special Ser/Thr-specific protein kinases are formed, which are known as cyclin-depen-dent kinases (CDKs). This term is used because they have to bind an activator protein (cyclin) in order to become active. At each control point in the cycle, specific CDKs associate with equally phase-specific cyclins. if there are no problems (e.g., DNA damage), the CDK-cyclin complex is activated by phosphorylation and/or dephosphorylation. The activated complex in turn phosphorylates transcription factors, which finally lead to the formation of the proteins that are required in the cell cycle phase concerned (enzymes, cytoskeleton components, other CDKs, and cyclins). The activity of the CDK-cyclin complex is then terminated again by proteolytic cyclin degradation. 

The rate of protein degradation also differs from enzyme to enzyme and depends on the conditions in the cell protein half-lives vary from a few minutes to many days. Some proteins are tagged for degradation in pro-teasomes (discussed in Chapter 28) by the covalent attachment of ubiquitin (recall the case of cyclin see Fig. 12-44). Some proteins are synthesized as inactive forms, or proenzymes, that become active only when a proteolytic event removes an inhibitory sequence in the proenzyme. 

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