The crystal structure of p13suc1, a p34cdc2-interacting cell cycle control protein.

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Abstract
P13suc1 binds to p34cdc2 kinase and is essential for cell cycle progression in eukaryotic cells. The crystal structure of S.pombe p13suc1 has been solved to 2.7 A 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 solution was the determination of the six selenium sites by direct methods. The structure is dominated by a four‐stranded beta‐sheet, with four further alpha‐helical regions. p13suc1 crystallizes as a dimer in the asymmetric unit stabilized by the binding of two zinc ions. A third zinc site stabilizes the higher‐order crystal packing. The sites are consistent with a requirement for zinc during crystal growth. A likely site for p13suc1‐protein interaction is immediately evident on one face of the p13suc1 surface. This region comprises a group of conserved, exposed aromatic and hydrophobic residues below a flexible negatively charged loop. A conserved positively charged area would also present a notable surface feature in the monomer, but is buried at the dimer interface. p13suc1 is larger than its recently solved human homologue p9CKS2, with the extra polypeptide forming a helical N‐terminal extension and a surface loop between alpha‐helices 3 and 4. Notably, p13suc1 does not show the unusual beta‐strand exchange that creates an intimate p9CKS2 dimer. p13suc1 cannot oligomerize to form a stable hexamer as has been proposed for p9CKS2.