Cell Biology Lecture No. 8: Mitosis & Cell Cycle Control II
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
• 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
• 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:
o Chromosome condensation (important for early phase),
o disassembly of nuclear envelope (lamins, nucleoporins, etc that give nucleus
structure are reorganized),
o interphase microtubule disassembly/mitotic spindle formation (break apart
interphase microtubules for activation of microtubules involved in segregating
An example of interphase microtubule disassembly is observed in the
rounding of HeLa cells during mitosis (due to changes in cytoskeleton). o remodelling of Golgi Body and ER (quiet down all the secretory mechanisms
that could interfere with mitosis).
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
• 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 MPF.
• Wee1 kinase adds a phos