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Lecture 8

Lecture 8: "Mitosis & Cell Cycle Control II"

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Western University
Biology 2382B
Robert Cumming

Cell Biology Lecture No. 8: Mitosis & Cell Cycle Control II th Monday February 4 , 2013 Wee1 & Cdc25 Regulating MPF Activity: -Wee1 is a kinase that phosphorylates CDK leading to its inactivation, while cdc25 is a phosphatase that removes the inhibitory phosphate group (left by wee1) from CDK and promotes its activation. In the recessive elongated fission yeast cells, these mutants were long and unable to engage mitosis (increased G2), because their mutation either resulted in an excess of wee1 or a deficit of cdc25 (leading to inappropriately inactive MPF). In the dominant premature fission yeast cells, these mutants were short and engaged mitosis unprepared (decreased G2), because their mutation either resulted in an excess of cdc25 or a deficit of wee1 (leading to prematurely activated MPF). - When mitotic cyclin binds to the CDK, it initially forms an inactive Mitosis Promoting Factor. Wee1, an inhibitory kinase, introduces a phosphate group to the tyrosine at position 15 (Y15) on bound CDK, so the MPF complex remains inactive (key to discouraging premature mitosis). In the next step, CAK (Cyclin Activating Kinase) phosphorylates the threonine at position 161 (T161) on bound CDK, so the MPF complex is somewhat activated (not optimal for engaging mitosis). It is only when the activating phosphatase cdc25 removes the phosphate on Y15 that the MPF is fully activated and ready to promote mitotic division. Functions Of Active MPF: -When the maximal activity of MPF is achieved, there are all sorts of functions that can be carried out with regards to mitosis: Chromosome condensation (important for early phase), disassembly of nuclear envelope (lamins, nucleoporins, etc that give nucleus structure are reorganized), interphase microtubule disassembly/mitotic spindle formation (break apart interphase microtubules for activation of microtubules involved in segregating chromosomes), and remodelling of Golgi Body and ER (quiet down all the secretory mechanisms that could interfere with mitosis). An example of interphase microtubule disassembly is observed in the rounding of HeLa cells during mitosis (due to changes in cytoskeleton). Exit From Mitosis: - It is important to shut off mitotic activation elements (as we don’t want to mimic the properties of cancer cells). Thus, the MPF is inactivated during anaphase through the process of ubiquitination (of cyclin specifically). The key enzyme involved is the APC/C (Anaphase-Promoting Complex/Cyclosome), which is a ubiquitin ligase that recognizes9 highly conserved amino acids (known as the destruction box) found in a number of cyclins. This is crucial for commencing rapid mitotic cyclin degradation. Regulation Of Mitotic Cyclin Levels & Overview Of Cell Cycle: -Note that CDK (cdc2) levels remains constant in the cell, but its activity changes (correlates with high activity of mitotic cyclin at the end of G2). From G1 to G2 there is an increase in the expression of mitotic cyclin and by late G2, the CDK binds with mitotic cyclin forming an inactive
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