THE CRYSTAL-STRUCTURE OF P13(SUC1), A P34(CDC2)-INTERACTING CELL-CYCLE CONTROL PROTEIN

p13(suc1) binds to p34(cdc2) kinase and is essential for cell cycle pr ogression in eukaryotic cells. The crystal structure of S.pombe p13(su c1) has been solved to 2.7 Angstrom resolution using data collected at the ESRF source, Grenoble, from both native crystals and crystals of a seleno-methionine derivative. The starting point for structure solut ion was the determination of the six selenium sites by direct methods. The structure is dominated by a four-stranded beta-sheet, with four f urther alpha-helical regions. p13(suc1) crystallizes as a dimer in the asymmetric unit stabilized by the binding of two zinc ions. A third z inc site stabilizes the higher-order crystal packing. The sites are co nsistent with a requirement for zinc during crystal growth. A likely s ite for p13(suc1)-protein interaction is immediately evident on one fa ce of the p13(suc1) surface. This region comprises a group of conserve d, exposed aromatic and hydrophobic residues below a flexible negative ly charged loop. A conserved positively charged area would also presen t a notable surface feature in the monomer, but is buried at the dimer interface. p13(suc1) is larger than its recently solved human homolog ue p9(CKS2), With the extra polypeptide forming a helical N-terminal e xtension and a surface loop between alpha-helices 3 and 4. Notably, p1 3(suc1) does not show the unusual beta-strand exchange that creates an intimate p9(CKS2) dimer. p13(suc1) cannot oligomerize to form a stabl e hexamer as has been proposed for p9(CKS2).