Vijayraghavan Sriram, Schwacha Anthony
Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA, 15260, USA.
Subcell Biochem. 2012;62:113-34. doi: 10.1007/978-94-007-4572-8_7.
In eukaryotes, the Mcm2-7 complex forms the core of the replicative helicase - the molecular motor that uses ATP binding and hydrolysis to fuel the unwinding of double-stranded DNA at the replication fork. Although it is a toroidal hexameric helicase superficially resembling better-studied homohexameric helicases from prokaryotes and viruses, Mcm2-7 is the only known helicase formed from six unique and essential subunits. Recent biochemical and structural analyses of both Mcm2-7 and a higher-order complex containing additional activator proteins (the CMG complex) shed light on the reason behind this unique subunit assembly: whereas only a limited number of specific ATPase active sites are needed for DNA unwinding, one particular ATPase active site has evolved to form a reversible discontinuity (gate) in the toroidal complex. The activation of Mcm2-7 helicase during S-phase requires physical association of the accessory proteins Cdc45 and GINS; structural data suggest that these accessory factors activate DNA unwinding through closure of the Mcm2-7 gate. Moreover, studies capitalizing on advances in the biochemical reconstitution of eukaryotic DNA replication demonstrate that Mcm2-7 loads onto origins during initiation as a double hexamer, yet does not act as a double-stranded DNA pump during elongation.
在真核生物中，Mcm2-7复合物构成了复制解旋酶的核心，复制解旋酶是一种分子马达，利用ATP结合和水解为复制叉处双链DNA的解旋提供能量。尽管Mcm2-7是一种环形六聚体解旋酶，表面上类似于来自原核生物和病毒的研究得更深入的同型六聚体解旋酶，但Mcm2-7是唯一已知的由六个独特且必需的亚基组成的解旋酶。最近对Mcm2-7和包含额外激活蛋白的高阶复合物（CMG复合物）的生化和结构分析揭示了这种独特亚基组装背后的原因：虽然DNA解旋只需要有限数量的特定ATP酶活性位点，但一个特定的ATP酶活性位点已进化形成环形复合物中的可逆间断（门）。S期期间Mcm2-7解旋酶的激活需要辅助蛋白Cdc45和GINS的物理结合；结构数据表明这些辅助因子通过关闭Mcm2-7门来激活DNA解旋。此外，利用真核生物DNA复制生化重建进展的研究表明，Mcm2-7在起始阶段作为双六聚体加载到起始位点，但在延伸阶段并不作为双链DNA泵起作用。
