Polyubiquitination marks eukaryotic proteins for degradation by proteasomes. Three enzymes are required for the Ubiquitin to work: E1, the Ubiquitin-activating enzyme; E2, the Ubiquitin-conjugating enzyme; and E3, the Ubiquitin ligase. SCF and APC/C are ubiquitin protein ligase complexes that that control three major transitions in the cell cycle: onset of S-phase through degradation of Sic1 [...]

Human Yeast Activates Overview CAK Kinase Cyclin-CDKs CAK positive phosphorylates CDK1, CDK2 and CDK4. CAK is itself a member of the CDK family and is composed of CDK7, cyclin H and an assembly protein Mat1. Checkpoint controls, inactivate Cdc25C and Cdc25a phosphatases to induce cell-cycle arrest. (link) Cdc25 phosphatase Cyclin-CDKs Involved in activating MPF. Cdc25a [...]

Human Yeast Inhibits Overview Wee1 Kinase Cyclin-CDKs Inhibitory phosphorylation of CDK1 and CDK2. (link INK4 Mid-G1 CDKs INK4s include cyclin-dependent kinase 4 and 6, which have important tumor suppression activity by inhibiting the mid G1 CDKs, thus inhibiting passage through G1. Both genes encoding INK4a are mutated in many human tumors, lessening their ability to [...]

Cycles in M-Phase Promoting Factor (MPF) activity control mitosis. As a protein kinase, MPF likely acts via phosphorylation of the major histone protein H1 and the major nuclear envelope protein lamin. This leads to the degradation of the nuclear envelope and condensation of chromatin into chromosomes in anticipation of mitosis. Found in all organisms, MPF [...]

Microtubule organizing centers (aka centrosomes) are composed of asters at each end, with centrosomes spanning between them. The tubules that connect to the chromosome kinetochore are called kinetochore microtubules, while the tubules which interact with each other are polar microtubules (aka non-kinetochore microtubules). Microtubules are composed of α- and β-tubulin monomers polymerized to form hollow [...]

Factor Overview E2F E2F is a transcription factor that by itself activates transcription. Rb Rb binds to E2F and represses its activation function. Rb is deactivated upon phosphorylation by mid G1 cyclin-CDKs (and, eventually, late G1 cyclin-CDKs). Cohesin Cohesin is a heterotrimeric complex of Smc1, Smc3 and Kleisin (Scc1). Smc1 and Smc3 form a circle [...]

Cyclin-dependent kinases regulate two important transitions in the eukaryotic cell cycle: entry into S-phase and entry into mitosis. CDKs are controlled at every level: presence (controlled degradation); activity (inhibitors); and even specificity (cyclins). Examples are shown below: Cyclins The specificity of cyclin-dependent kinases is regulated by their cyclins. For example, S. cerevisiae uses the same [...]