BIOL 2020 Lecture Notes - Lecture 30: Nuclear Membrane, P53, Apoptosis

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17 Apr 2018

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Tyr15 (stop: cdc needs to be in contact with cyclin, no g2 phase, cell can"t put the breaks there in wee1- If tyr15 is phosphorylated by wee1, but phosphate is not taken away you will end up with cdc 25: dephosphorylation of cdk tyrosine 15 by cdc-25 causes mitosis to occur. Other regulators of cdk activity: additional kinases and phosphatases (influence levels of cak and such, controlled proteolysis, ubiquitin proteasome pathway, apc: anaphase promoting complex (ubiquitin ligases, exit of mitosis to g1) Subcellular localization (before cyclin can interact with cdk, it needs to be allowed to accumulate in the nucleus, when g2 starts and progresses export signal is inhibited: cyclin b1 accumulates in nucleus. If too much damage from ionizing radiation, cell will undergo apoptosis. Mitosis: during m-phase all the cell"s energy is devoted to mitosis, there is no response to external stimuli (makes it vulnerable)

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For your responses please justify your answer. No justificationfor answer will recieve "needs work" rating.

32. Which statements concerning the regulation of CDK/cyclincomplexes are correct?
A. At different points in the cell cycle, G1 and mitotic cyclinscan activate the same CDK subunit.
B. Phosphorylation of CDK proteins can both activate and inhibitcyclin/CDK activity.
C. If CAK has phosphorylated the CDK subunit, CDKs will remainactive in the absence of cyclin.
D. The CDK/G1 cyclin complexes that control S phase initiation arelikely to have signal sequences
that target them for nuclear import.
F. Cyclin/CDK complexes are frequently targets of cell cyclecheckpoint pathways.

33. Which of the following statements concerningubiquitin-mediated proteolysis are correct?
A. Ubiquitin modification serves as a molecular âtagâ to signaldegradation by the proteosome.
B. Both G1 and mitotic cyclins are targets for ubiquitin-mediatedproteolysis.
C. Nuclear envelope breakdown during mitosis is accomplished byubiquitin-mediated
proteolysis of lamin intermediate filament proteins.
D. The separation of sister chromatids at the metaphase to anaphasetransition is controlled by
ubiquitin-mediated proteolysis of an anaphase inhibitor.
E. Conditional mutations that inactivate the gene encodingubiquitin in yeast would be expected
to exhibit a cdc phenotype.

QUESTION 1:

(a)Examine the Figure below relating to the S.pombe cellcycle:

1) Describe the function of Cdc25 and Wee1and why deficit of Cdc25 or excess of Wee1 can cause G2arrest

2) b) Describe how deficit of Wee1 or excess ofCdc25 may result in abnormally small cells

(b) Theactivities of Wee1 tyrosine kinase and Cdc25 tyrosine phospatasedetermine the state of phosphorylation of tyrosine 15 in the Cdk1component of mitotic cyclin-Cdk complexes (MPF). When tyrosine 15is phosphorylated, M-Cdk is inactive; when tyrosine 15 is notphosphorylated, M-Cdk is active. Just as the activity of M-Cdkitself is controlled by phosphorylation, so too are the activitiesof Wee1 kinase and Cdc25 phosphatase. The regulation of thesevarious activities can be studied in extracts of frog oocytes. Insuch extracts Wee1 tyrosine kinase is active and Cdc25 tyrosinephosphatase is inactive. As a result M-Cdk is inactive because itsCdk1 component is phosphorylated on tyrosine 15. M-Cdk in theseextracts can be rapidly activated by addition of okadaic acid,which is a potent inhibitor of serine/threonine phosphatases. Usingantibodies specific for each component it is possible to examinetheir phosphorylation states by changes in mobility upon gelelectrophoresis.

(Phosphorylated proteins generally run slower than theirnon phosphorylated counterparts).

1) Based on theresults with okadaic acid, decide whether the active forms of Wee1and Cdc25 are phosphorylated or unphosphorylated. In the figureindicate the phosphorylated forms of Wee 1 and Cdc25 and label thearrows connecting their active and inactive forms to show whichtransitions are controlled by protein kinases and which by proteinphosphatases.

2) Are the proteinkinases and phosphatases that control Wee1 and Cdc25 specific forserine/threonine side chains or for tyrosine side chains? How doyou know?

3) How doesaddition of okadaic acid cause an increase in phosphorylation ofWee1 and Cdc25, but a decrease in phosphorylation ofCdk1?

4) If you assumethat Cdc25 and Wee1 are targets for phosphorylation by activeM-Cdk, can you explain how the appearance of a small amount ofactive M-Cdk would lead to its rapid and completeactivation?

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BIOL 2020 Lecture Notes - Lecture 30: Nuclear Membrane, P53, Apoptosis

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