MCB 252 Topic 32 Cell Cycle RegulationProf David Rivier MCB 252 Spring 2015 Components CDK kinase subunit Cdc28 G1 S cyclins Cln3 Cln1 Cln2 S phase cyclins Clb5 Clb6 M phase cyclins Clb 1 Clb2 Clb3 Clb4 Sic1 Inhibits S phase CDKs SCF Ub ligase degradation of Sic1 APC C Cdh1 Ub ligase degradation of S and M CDKs APC C Cdc20 Ub ligase onset of anaphase Wee 1 Kinase that activates M CDKS Cdc25 Phosphatase that inactivates M CDKs Whi5 Inhibits txn of G1 S and S cyclins Cdc14 Phosphatase reqd for exit from mitosis Logic The cell cycle is regulated by a series of irreversible switches Once a switch is turned off it cannot be turned back on until the next cell cycle Each type of CDK is switched on and off G1 S CDKs S CDKs M CDKs and APC C are turned on and off at specific times in the cycle APC C targets S and M CDKS for degradation Logic of Regulation of the Cell Cycle Each CDK activates events required for it s stage of the cycle Each CDK triggers events that lead to the activation of the next CDK or APC C Each CDK triggers events that lead to it s own inactivation Components CDK kinase subunit Cdc28 G1 S cyclins Cln3 Cln1 Cln2 S phase cyclins Clb5 Clb6 M phase cyclins Clb 1 Clb2 Clb3 Clb4 Sic1 Inhibits S phase CDKs SCF Ub ligase degradation of Sic1 APC C Cdh1 Ub ligase degradation of S and M CDKs APC C Cdc20 Ub ligase onset of anaphase Wee 1 Kinase that activates M CDKS Cdc25 Phosphatase that inactivates M CDKs Whi5 Inhibits txn of G1 S and S cyclins Cdc14 Phosphatase reqd for exit from mitosis Nutrients Translation and Stabilization of initial G1 S cyclin Initial G1 S CDK Txn of additional G1 S Cyclin Genes Full Activation of G1 S CDKs Cdc14 phosphatase resets Whi5 at the end of mitosis Later CDKs keep Whi5 and Rb phosphorylated until next G1 Gene encoding Rb is mutated LOF in many cancers START G1 S CDKs shut off the machinery that degrades S M cyclins S phase cyclins produced in G1 but held in inactive form As level of G1 S CDK peaks START activation of S CDKs SCF Ub ligase 1 S CDK phosphorylate G1 S cyclin degradation of G1 S cyclin M CDK also targets G1 S cyclin for degradation 2 S CDK replication 3 S CDK txn of M cyclins S CDKs and M CDKs phosphorylate G1 S cyclin targeting it for degradation G1 S cyclins can t accumulate again until S and M CDKs gone G1 S CDKs inactivate APC which degrades S and M cyclins M cyclins are produced in S phase S CDKs activate transcription of genes that code for the M cyclins Mitotic CDKs initially held in inactive form via phosphorylation of the M CDK by Wee1 This figure violates standard rules of genetic regulation diagrams This modification is correct We don t yet understand all aspects of M CDK activation but one way to think about it is Initially the amount of Wee1 is greater than the amount of M CDK Eventually the amount of MCDK becomes greater than Wee1 and M CDK becomes active Activation of M CDK activates the phosphatase and inactivates the kinase resulting in rapid activation of the entire pool of M CDK APC E3 ubiquitin ligase M CDKs activate APC C Cdc20 APC C Cdc20 function is required for sister chromatid separation APC C Cdc20 targets S cyclins for degradation APC C Cdc20 targets a large fraction of the M cyclins for degradation APC C Cdh1 targets the remainder of the M cyclins for degradation Activation of APC C Cdh1 Cdc14 is a phosphatase Activation of Cdc14 is required for exit from mitosis Cdc14 activates APC C Cdh1 which targets remaining M cyclins for degradation which results in activation of Sic1 via dephosphorylation This resets Sic1 to it s G1 state We ll talk about how Cdc14 gets activated later Cdc14 dephosphorylates Whi5 resetting Whi5 to it s early G1 form Cdc14 leads to repression of txn of M cyclins APC C activity persists into G1 G1 S CDKs shut of APC C allowing accumulation of S cyclins Destruction of M cyclins paves the way for accumulation of the G1 S cyclins G1 S CDKs inactivate APC C Cdh1 thereby allowing accumulation of Sphase CDKs Overview Regulation of DNA Replication Regulation of anaphase sister chromatid separation Other events in M Checkpoints Cancer Logic of Regulation of the Cell Cycle Each CDK activates events required for it s stage of the cycle Each CDK triggers events that lead to the activation of the next CDK Each CDK triggers events that lead to it s own inactivation and sometimes to the inactivation of the previous CDK M G1 S Initiation is regulated so it occurs once and only once per cell cycle Can only build pre RC in G1 Initiation in S phase disassembles the pre RC and it can t be re assembled until the next G1 Pre RC assembly prevented in M Lecture Overview Regulation of DNA Replication Regulation of anaphase sister chromatid separation Other events in M Checkpoints Cancer Cleavage of Cohesins at Onset of Anaphase Many M phase events regulated by protein phosphorylation Breakdown of nuclear lamins Assembly of spindles Breakdown of nuclear envelope Breakdown of nuclear pores etc And re set by protein de phosphorylation via cdc14 and the MEN The events of early M must be reversed in late M From MCB 150 Chromosome decondensation Fusion of vesicles Assembly of nuclear lamina and nuclear pore complexes Reassembly of nuclear envelope and nuclear pores signaled in part by RAN Mystery of the nuclear envelope Lecture Overview Regulation of DNA Replication Regulation of anaphase sister chromatid separation Other events in M Checkpoints Cancer Cell Cycle Checkpoints Inhibitors of Progression Inhibit Downstream Stages Until the Critical Steps Completed Steve Elledge DNA Damage Checkpoint Checkpoints and Cancer Loss of Checkpoints loss of brakes on cycle excess proliferation loss of cell death increased genomic instability Checkpoints Insure Faithful Transmission of DNA Genetic Material Some act when there is a problem DNA damage Stalled Replication Forks Some act to prevent cell cycle transitions until previous step is complete Ongoing replication Spindle assembly checkpoint Spindle position checkpoint Spindle position checkpoint in yeast Activation of Cdc14 and the MEN does not occur until the spindles are properly localized Checkpoints Brakes on the Cell Cycle Checkpoints insure Faithful Replication Repair and Segregation of Chromosomes Loss of Checkpoints Genomic Instability Some Hallmarks of Cancer Cells 1 Unregulated Proliferation 2 Genomic Instability increased mutation rate 3 Metastasis often sessile motile Many Types of Cancer 1 Different Cancers Different Genes Mutated 2 Approximately 6 7