Bourne Y, Watson M H, Arvai A S, Bernstein S L, Reed S I, Tainer J A
Centre National de la Recherche Scientifique, Marseille, France.
Structure. 2000 Aug 15;8(8):841-50. doi: 10.1016/s0969-2126(00)00175-1.
The Saccharomyces cerevisiae protein Cks1 (cyclin-dependent kinase subunit 1) is essential for cell-cycle progression. The biological function of Cks1 can be modulated by a switch between two distinct molecular assemblies: the single domain fold, which results from the closing of a beta-hinge motif, and the intersubunit beta-strand interchanged dimer, which arises from the opening of the beta-hinge motif. The crystal structure of a cyclin-dependent kinase (Cdk) in complex with the human Cks homolog CksHs1 single-domain fold revealed the importance of conserved hydrophobic residues and charged residues within the beta-hinge motif.
The 3.0 A resolution Cks1 structure reveals the strict structural conservation of the Cks alpha/beta-core fold and the beta-hinge motif. The beta hinge identified in the Cks1 structure includes a novel pivot and exposes a cluster of conserved tyrosine residues that are involved in Cdk binding but are sequestered in the beta-interchanged Cks homolog suc1 dimer structure. This Cks1 structure confirms the conservation of the Cks anion-binding site, which interacts with sidechain residues from the C-terminal alpha helix of another subunit in the crystal.
The Cks1 structure exemplifies the conservation of the beta-interchanged dimer and the anion-binding site in evolutionarily distant yeast and human Cks homologs. Mutational analyses including in vivo rescue of CKS1 disruption support the dual functional roles of the beta-hinge residue Glu94, which participates in Cdk binding, and of the anion-binding pocket that is located 22 A away and on an opposite face to Glu94. The Cks1 structure suggests a biological role for the beta-interchanged dimer and the anion-binding site in targeting Cdks to specific phosphoproteins during cell-cycle progression.
酿酒酵母蛋白Cks1（细胞周期蛋白依赖性激酶亚基1）对细胞周期进程至关重要。Cks1的生物学功能可通过两种不同分子组装形式之间的转换进行调节：一种是单结构域折叠，由β-铰链基序关闭形成；另一种是亚基间β-链互换二聚体，由β-铰链基序打开产生。细胞周期蛋白依赖性激酶（Cdk）与人Cks同源物CksHs1单结构域折叠形成复合物的晶体结构揭示了β-铰链基序内保守疏水残基和带电残基的重要性。
分辨率为3.0 Å的Cks1结构揭示了Cksα/β核心折叠和β-铰链基序的严格结构保守性。在Cks1结构中鉴定出的β-铰链包含一个新的枢轴，并暴露了一簇保守的酪氨酸残基，这些残基参与Cdk结合，但在β-互换的Cks同源物suc1二聚体结构中被隔离。该Cks1结构证实了Cks阴离子结合位点的保守性，该位点在晶体中与另一个亚基C末端α螺旋的侧链残基相互作用。
Cks1结构体现了在进化距离较远的酵母和人Cks同源物中β-互换二聚体和阴离子结合位点的保守性。包括体内拯救CKS1破坏在内的突变分析支持β-铰链残基Glu94的双重功能作用，其参与Cdk结合，以及位于距Glu94 22 Å且在其相对面上的阴离子结合口袋的双重功能作用。Cks1结构表明β-互换二聚体和阴离子结合位点在细胞周期进程中将Cdk靶向特定磷酸化蛋白方面具有生物学作用。
