OLIGOMERIZATION STATE IN SOLUTION OF THE CELL-CYCLE REGULATORS P13(SUCL) FROM THE FISSION YEAST AND P9(CKSPHY) FROM THE MYXOMYCETE PHYSARUM, 2 MEMBERS OF THE CKS FAMILY

The cks proteins (for cdc2 kinase subunit) are essential cell cycle re gulators, They interact strongly with the mitotic cdc2 kinase, but the mechanism and the biological function of this association still await understanding, The oligomerization state in solution of two members o f this ubiquitous protein family, the suc1 gene product from the fissi on yeast and the newly cloned cksphy gene product from the myxomycete Physarum, was investigated by small-angle X-ray scattering (SAXS) and biochemical methods, We found that the major molecular species are mon odispersed monomeric proteins. Minor amounts of dimeric suc1 proteins were also found, but no equilibrium between the two forms was observed and surprisingly, the hexameric assemblies observed in the crystal st ructure of the human ckshs2 homolog were not detected, These apparent discrepancies between proteins that display cross-complementation addr ess the question of the control of the cks oligomerization process and its link to the biological function.