CELL CYCLE PROTEINS. M.E. Noble*, J.A. Endicott*, N.R. Brown*, E. Garman*, R. Dzivenu*, A. Lawrie*, L.N. Johnson*, P. Nurse, and R.T. Hunt⁺.*Laboratory of Molecular biophysics, South Parks Road, Oxford, U.K., ⁺ICRF, Clare Hall Labs., South Mimms, Herts, U.K., = ICRF, Lincoln's Inn Fields, London, U.K

Progress through the cell cycle is regulated by the "Cyclin Dependent Kinases" (CDKs). Different CDKs are activated at appropriate times by association with cognate "cyclins", and by phosphorylation of a conserved threonine. CDKs may also be found in complex with other proteins, such as p13^suc1 and the CDK inhibitors. Our aim is to study proteins of the cell cycle, and to understand the structural principles behind their actions and interactions. This study has produced the crystal structures of p13^suc1, and of an active 260 residue fragment of cyclin A (cyclin A3)[2].

Although p13^suc1 is essential for successful passage through the cell cycle, its mode of action is not known. Our structure of p13^suc1 reveals a compact, principally beta-sheet fold. It differs from the structure of a human homologue of p13^suc1, and from another structure of p13^suc1 itself [3], where details of the protein purification and crystallisation were different. These proteins formed dimers, in which beta strands from distinct monomers interlace to form a beta-sheet like that observed in our structure. We are currently producing mutants to probe the function of p13^suc1.

Cyclin A is first observed late in the G1 phase of the cell cycle, and its abundance increases throughout S and G2. It forms complexes with either CDK2 or CDC2. Our structure of a genetically truncated form of cyclin A (residues 171-432) consists of two strikingly similar helical domains, despite low sequence homology. Comparison with the structure of a complex between a fragment of cyclin A and CDK2 [4] showed that in contrast to the major structural changes of CDK2, cyclin remains essentially unaltered when complexed. Thus, cyclin acts as a template, complimentary to the basally active conformation of CDK2 in the complexed structure.

Our poster will describe details of the results mentioned above, as well as the results of on-going work to further characterise these proteins.

[1] Endicott et al., EMBO J. 14, 1004. [2] Brown et al., Structure 3, 1235

[3] Bourne et al., PNAS 92, 10232. [4] Jeffrey et al., Nature 376, 313.