Hirano Michiko, Hirano Tatsuya
Cold Spring Harbor Laboratory, One Bungtown Road, PO Box 100, Cold Spring Harbor, NY 11724, USA.
EMBO J. 2002 Nov 1;21(21):5733-44. doi: 10.1093/emboj/cdf575.
Structural maintenance of chromosomes (SMC) proteins play central roles in regulating higher order chromosome dynamics from bacteria to humans. As judged by electron microscopy, the SMC homodimer from Bacillus subtilis (BsSMC) is composed of two antiparallel, coiled-coil arms with a flexible hinge. Site-directed cross-linking experiments show here that dimerization of BsSMC is mediated by a hinge-hinge interaction between self-folded monomers. This architecture is conserved in the eukaryotic SMC2-SMC4 heterodimer. Analysis of different deletion mutants of BsSMC unexpectedly reveals that the major DNA-binding activity does not reside in the catalytic ATPase domains located at the ends of a dimer. Instead, point mutations in the hinge domain that disturb dimerization of BsSMC drastically reduce its ability to interact with DNA. Proper hinge function is essential for BsSMC to recognize distinct DNA topology, and mutant proteins with altered hinge angles cross-link double-stranded DNA in a nucleotide-dependent manner. We propose that the hinge domain of SMC proteins is not a simple dimerization site, but rather it acts as an essential determinant of dynamic SMC-DNA interactions.
染色体结构维持（SMC）蛋白在调控从细菌到人类的高阶染色体动态变化中发挥着核心作用。通过电子显微镜判断，来自枯草芽孢杆菌的SMC同型二聚体（BsSMC）由两条反平行的卷曲螺旋臂和一个柔性铰链组成。定点交联实验表明，BsSMC的二聚化是由自折叠单体之间的铰链-铰链相互作用介导的。这种结构在真核生物的SMC2-SMC4异源二聚体中是保守的。对BsSMC不同缺失突变体的分析意外地发现，主要的DNA结合活性并不存在于位于二聚体末端的催化ATP酶结构域中。相反，铰链结构域中干扰BsSMC二聚化的点突变会大幅降低其与DNA相互作用的能力。适当的铰链功能对于BsSMC识别不同的DNA拓扑结构至关重要，并且具有改变铰链角度的突变蛋白会以核苷酸依赖的方式交联双链DNA。我们提出，SMC蛋白的铰链结构域并非简单的二聚化位点，而是动态SMC-DNA相互作用的关键决定因素。
