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Laboratory of Molecular Biophysics
Laboratory Journal 2001
Dr. J. A. Endicott

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Jane A. Endicott

Cell Cycle Proteins


Sequential activation of members of the cyclin-dependent protein kinase (CDK) family promotes the correct timing and ordering of events required for cell growth and cell division (reviewed in [1]). In addition to driving progress through the cell cycle, CDKs are also the downstream targets of checkpoint pathways (reviewed in [2]). These checkpoints act to ensure that critical cell cycle events have been successfully completed before the cell progresses into the next cell cycle stage. They are composed of a surveillance system that detects when a particular cell cycle event has not been correctly executed and a signal transduction pathway whose ultimate target can be a CDK. Monomeric CDKs are inactive and require both association with a positive regulatory subunit, called a cyclin, and phosphorylation on a conserved threonine residue that lies within the activation loop for full activity (reviewed in [3, 4]). Both the CDK and cyclin families have multiple members, but only CDKs 1, 2, 4 and 6, when bound to their cognate cyclins, appear to have major roles in controlling cell cycle progression (reviewed in [1]). These CDK/cyclin complexes are then additionally controlled by mechanisms that include inhibitory phosphorylation, protein association, subcellular localisation and targeted destruction of regulatory proteins (reviewed in [1, 5, 6, 7]).

This year we have continued our studies on the structural consequences of CDK phosphorylation by characterising a complex of CDK2/cyclin A phosphorylated on Tyr15 and Thr160 (Y15pT160pCDK2) bound to a substrate-trapping mutant of Cdc25A. A second research area is to characterise interactions between CDKs and their regulatory molecules. This year we have crystallised a ternary T160pCDK2/cyclin A/Cks1 complex and have continued to work towards a molecular description of the structure and function of a Skp1/Skp2/CDK2/cyclin A complex.

An increasingly important mode of cell cycle control appears to be the targeted ubiquitination of cell cycle regulatory proteins, their directed degradation often being responsible for triggering key events in cell cycle progression [8, 9]. At least three major cell cycle transitions require specific degradation of regulatory proteins: entry into S- phase, separation of sister chromatids and exit from mitosis. Ubiquitin-dependent degradation pathways can consist of three enzymes/enzyme complexes, the last complex in the pathway (the E3) being important for substrate selection. E3s are diverse and are frequently multi-protein complexes. Two types of E3 that regulate cell cycle progression have been well characterised: SKP1, cullin, F-box complexes (SCFs) responsible for regulatory events at the G1-S transition [9], and the anaphase promoting complex (APC) that acts at the end of mitosis and controls passage back into the G1 phase of the cell cycle [10]. This year we have continued structural and functional characterisation of SCFSkp2 and the APC.

Aberrant CDK activity is a common defect in a variety of human tumours. We continue to have an interest in the determination of structures of CDK2 and CDK2/cyclin A in complex with various small molecules with the aim of designing potent and selective CDK inhibitors.

Tyr15pThr160pCDK2/cyclinA/Cdc25A complex.

J. Tucker and A. Cheung

Skp1/Skp2/CDK2/cyclin A and Skp1/Skp2/CDK2/cyclin A/Cks1 complexes

N. Schueller

p13suc1 and CDK2/cyclin A/HsCks1 complexes

S. Holton


N. Schueller and J. Gruber.

Anaphase promoting complex.

I. Taylor and E. Dubinina.


S. Holton and D. Burgess

CDK/inhibitor complexes

T. Davies and D. Pratt

Acknowledgements and References

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