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