Cyclins are synthesized and degraded in a cyclic manner and with correlation to the cell cycle

ProteinLevel

Time

cyclin A cyclin B

M M M

Something needs to go away in order for the cell cycle to proceed

sea urchin!sea urchin!

Cyclins are indeed degraded

Yeast genetics Needed for promoting cells through the cell cycle

CDK

Biochemistry in sea urchinAppear in correlation with the cell cycle

Cyclin

Time to bring them together

Overview of the frog life cycle

1 mm

spermtadpole feeds, grows

and becomes an adult frog

OOCYTE GROWS WITHOUT DIVIDING(MONTHS) FERTILIZATION

FERTILIZED EGG DIVIDES WITHOUT GROWING(HOURS)

Xenopus laevis

The maturation of frog eggs

Progesterone

8 months!

Yoshio Masui, 1971The Maturation of Frog Eggs

Progesterone

MPF = Maturation Promoting Factor

Injections of M-phase mitotic cells from different organisms also promoted Xenopus oocyte maturation, showing that MPF is a general factor in promoting mitosis

MPF activity peaks right before mitosis and drops before mitosis is completed

Checking MPF activity from different cells and different stages

Purification of MPFThe birth of cyclin dependent kinases

MPF is a heterodimer of CDK and cyclin

Cyclin-CDK complexes control the cell cycle clock

MPF promotes entry to mitosis and then disappears

Now performsa cell cycle function

CDKs form heterodimers with cyclins and become active kinases

Yeasts have one CDK and several cyclinsHumans have 4 CDKs and 4 cyclins

MPF is a heterodimer of CDK and cyclin active in the entry into mitosis

- Cyclin-Cdk complexes function in different phases

- G1/S-Cdk complexes commit the cell to a new cell cycle

- S-Cdk complexes promote S phase

- M-Cdk complexes allow entry into mitosis

- M-Cdk complexes are removed before anaphase

X

Example: cycB-Cdk1 appears in mitosis, phosphorylates lamin and leads to nuclear envelope

breakdown during early mitosis

cycB-Cdk1 will be degraded during mitosis to allow formation of a new nuclear envelop breakdown during telophase

The Nobel Prize in Physiology or Medicine, 2001“For their discovery of key regulators of the cell cycle”

CDKs are the major activators/inhibitors controlling the cell cycle

How are they regulated?

Mechanisms of CDKs regulation

1. Abundance of cyclins

2. CDK phosphorylation

3. Binding to CKIs (inhibitory proteins)

CDK

Cyclin

active

p21+

inactiveCDK

Cyclinp21

ProteinLevel

Time

cyclin A cyclin B

M M M

Mechanisms of CDKs regulation

1. Abundance of cyclins- cyclins need to appear- cyclins need to disappear

A destruction box targets degradation of the cyclin

Destruction is achieved through ubiquitination

Mutations in the box = no degradation = cyclins always there = cell cycle cannot be completed

Ubiquitination

Proteasome

E1, E2, and E3 are all important

E3 ligase is the proteins that conferss specificity to the

target