1. Cyclins are synthesized and degraded in a cyclic manner and with correlation to the cell cycle Protein Level Time cyclin A cyclin B MMM Something needs to go away in order for the cell cycle to proceed

2. sea urchin! Cyclins are indeed degraded

3. Yeast genetics Needed for promoting cells through the cell cycle CDK Biochemistry in sea urchin Appear in correlation with the cell cycle Cyclin Time to bring them together

4. Overview of the frog life cycle 1 mm sperm tadpole feeds, grows and becomes an adult frog OOCYTE GROWS WITHOUT DIVIDING (MONTHS) FERTILIZATION FERTILIZED EGG DIVIDES WITHOUT GROWING (HOURS) Xenopus laevis

5. The maturation of frog eggs Progesterone 8 months!

6. Yoshio Masui, 1971 The Maturation of Frog Eggs Progesterone

7. 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

8. MPF activity peaks right before mitosis and drops before mitosis is completed Checking MPF activity from different cells and different stages

10. Purification of MPF The birth of cyclin dependent kinases

11. MPF is a heterodimer of CDK and cyclin Cyclin-CDK complexes control the cell cycle clock MPF promotes entry to mitosis and then disappears

12. Now performs a cell cycle function CDKs form heterodimers with cyclins and become active kinases

13. Yeasts have one CDK and several cyclins Humans have 4 CDKs and 4 cyclins

14. MPF is a heterodimer of CDK and cyclin active in the entry into mitosis - 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 X

15. 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

16. The Nobel Prize in Physiology or Medicine, 2001 “For their discovery of key regulators of the cell cycle”

17. CDKs are the major activators/inhibitors controlling the cell cycle How are they regulated?

18. Mechanisms of CDKs regulation 1. Abundance of cyclins 2. CDK phosphorylation 3. Binding to CKIs (inhibitory proteins) CDK Cyclin active p21 + inactive CDK Cyclin p21

19. Protein Level Time cyclin A cyclin B MMM Mechanisms of CDKs regulation 1.Abundance of cyclins - cyclins need to appear - cyclins need to disappear

20. A destruction box targets degradation of the cyclin Destruction is achieved through ubiquitination Mutations in the box = no degradation = cyclins always there = cell cycle cannot be completed

21. Ubiquitination Proteasome E1, E2, and E3 are all important E3 ligase is the proteins that conferss specificity to the target