Cell Cycle
Sequential series of events from one mitosis to the next
Regulated at critical transition points: G1/S and G2/M
Regulation is essential for growth control and for genetic integrity
Cell Cycle - Overview
The G2/M Transition Occurs when cell cycle events are “complete” Requires intact genome, replicated
chromosomes, accumulation of critical factors mediating exit from interphase - prophase and entry to M phase.
MPF (“metaphase-promoting factor,” a.k.a., “mitosis-promoting factor,” a.k.a. “maturation-promoting factor”) is universal regulator of this important transition
End-point of G2/M = Metaphase
How do we know that MPF is essential? Xenopus oocyte “meiotic maturation” - a model
system MPF in other cells M-phase regulation revealed by yeast mutants
Saccharomyces cerevisiae - CDC28 Schizosaccharomyces pombe - cdc2
“Intersecting cell cycles” - topic for journal critique, Tuesday 16 April
Xenopus - Meiotic Maturation
MPF Activity Cycles
Further Evidence for Universal “MPF”
Budding Yeast Cell Division Cycle (cdc) Mutants
Fission Yeast cdc Mutants
Evidence for “Cyclin” Activity
Sea urchin (yet another model!): a specific protein cycles in quantity during cleavage divisions of the embryo
This was designated “cyclin” Subsequently, it was shown that this
protein co-purifies with MPF activity, but that it is not the sole component of MPF
MPF = cyclin B + p34 (CDC28/cdc2)
Phosphorylation and Dephosphorylation Regulate MPF Activity
Y15 - inhibitory phosphorylation by wee1/mik1 kinases Active site for ATP binding
T161 - phosphorylated by Cdk7/CycH (CAK) This residue occludes substrate-binding site
until it is phosphorylated Y15 - active dephosphorylation by Cdc25
protein phophatase
Kinases and Phosphatases
Missing Pieces: MPF/Cdk Substrates
Dramatic increase in phosphorylated proteins - MPM2 antigens
Autophosphorylation of cdc2 *Histone H1 phosphorylation assay (physiological
significance?) Histone H3 phosphorylation on ser10 may be more
significant Spatially and temporally correlated with chromosome
condensation Nuclear lamins - phosphorylation leads to solubilization
(breakdown of nuclear envelope)
How do cells get out of metaphase?
Via anaphase by action of the anaphase promoting complex (APC)
Injection into Xenopus oocytes of RNA encoding non-degradable cyclin B leads to sustained high MPF and metaphase arrest
Ubiquitination of chromosome cohesins and cyclin targets them to the proteosome
Synthesis - Destruction Cycle
Other Cell Cycle Kinases MPF - Cdk1 = “maestro of M phase NIMA (“never in mitosis”) family of
kinases (Aspergillus nidulans) Polo family kinases (Drosophila
melanogaster) Aurora family kinases All implicated in control of centrosome
and spindle poles
Multiple Cdk-Cyclin Complexes Regulate Cell-Cycle Transitions
G1 inactivation of APCG1/SG2/M
Models for Cdk Regulation