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Download Cell Cycle II
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Cyclins are synthesized and degraded in a cyclic manner and with correlation to the cell cycle Protein Level cyclin A cyclin B Time M M M Something needs to go away in order for the cell cycle to proceed Cyclins are indeed degraded sea urchin! sea urchin! CDK Yeast genetics Needed for promoting cells through the cell cycle Cyclin Biochemistry in sea urchin Appear in correlation with the cell cycle Time to bring them together Overview of the frog life cycle Xenopus laevis OOCYTE GROWS WITHOUT DIVIDING (MONTHS) FERTILIZATION FERTILIZED EGG DIVIDES WITHOUT GROWING (HOURS) 1 mm sperm tadpole feeds, grows and becomes an adult frog The maturation of frog eggs Progesterone 8 months! The Maturation of Frog Eggs Yoshio Masui, 1971 Progesterone MPF = Maturation Promoting Factor Injections of M-phase mitotic cells from different organisms also promoted Xenopus oocyte maturation, showing that MPF is a general factor in promoting mitosis Checking MPF activity from different cells and different stages MPF activity peaks right before mitosis and drops before mitosis is completed Purification of MPF The birth of cyclin dependent kinases MPF is a heterodimer of CDK and cyclin MPF promotes entry to mitosis and then disappears Cyclin-CDK complexes control the cell cycle clock CDKs form heterodimers with cyclins and become active kinases Now performs a cell cycle function Yeasts have one CDK and several cyclins Humans have 4 CDKs and 4 cyclins MPF is a heterodimer of CDK and cyclin active in the entry into mitosis X - Cyclin-Cdk complexes function in different phases - G1/S-Cdk complexes commit the cell to a new cell cycle - S-Cdk complexes promote S phase - M-Cdk complexes allow entry into mitosis - M-Cdk complexes are removed before anaphase Example: cycB-Cdk1 appears in mitosis, phosphorylates lamin and leads to nuclear envelope breakdown during early mitosis cycB-Cdk1 will be degraded during mitosis to allow formation of a new nuclear envelop breakdown during telophase The Nobel Prize in Physiology or Medicine, 2001 “For their discovery of key regulators of the cell cycle” CDKs are the major activators/inhibitors controlling the cell cycle How are they regulated? Mechanisms of CDKs regulation 1. Abundance of cyclins 2. CDK phosphorylation 3. Binding to CKIs (inhibitory proteins) active inactive CDK CDK Cyclin + p21 Cyclin p21 Mechanisms of CDKs regulation 1. Abundance of cyclins - cyclins need to appear - cyclins need to disappear Protein Level cyclin A cyclin B Time M M M A destruction box targets degradation of the cyclin Mutations in the box = no degradation = cyclins always there = cell cycle cannot be completed Destruction is achieved through ubiquitination Ubiquitination E1, E2, and E3 are all important E3 ligase is the proteins that conferss specificity to the target Proteasome
