Investigating the structural assembly of RNR multimers

描述（由申请人提供）：核糖核苷酸还原酶（RR）是维持生命的关键，因为它通过将核糖核苷酸还原为脱氧核糖核苷酸来催化从头合成DNTP的最慢步骤。RR是一种多亚基酶，由含有两个变构位点和一个催化位点的亚基和含有启动醇基催化的自由基的b亚基组成。近40年来，人们一直认为逆转录酶以异源四聚体的形式存在。然而，这个组织未能解释dATP如何失活而ATP如何激活酶。最近，这一观点受到了来自其他实验室的生化数据和我们报告的结构数据的挑战，这些数据支持其激活剂ATP和抑制剂dATP诱导的高阶低聚物的存在。有趣的是，虽然ATP是激活剂，而dATP是RR的抑制剂，但它们都导致亚基六聚体化。我们认为这两种六聚体具有不同的包装排列，从而导致相反的RR活性结果。此外，已知两种重要的抗癌药物吉西他滨和氯法拉滨可与高阶RR低聚物结合。因此，为了了解RR在真核生物中是如何被调控并被抗癌药物靶向，阐明RR的结构是至关重要的

DESCRIPTION (provided by applicant): Ribonucleotide reductase (RR) key to maintaining life as it catalyzes the slowest step in de novo DNTP synthesis by reducing ribonucleotides to deoxy ribonucleotides. RR is a multi-subunit enzyme consisting of a subunit that contains two allosteric sites and a catalytic site, and a b subunit that houses a free-radical that initiates thol-based catalysis. For almost 40 years RRs were thought to exist as heterotetramers. This organization failed, however, to explain how dATP inactivates and ATP activates the enzyme. Recently, this view has been challenged by biochemical data from other labs and structural data reported by us that support the existence of higher-order oligomers induced by its activator ATP and its inhibitor dATP. Interestingly, though ATP is an activator and dATP is an inhibitor of RR, they both cause the subunit to hexamerize. We propose that the two hexamers have different packing arrangements which lead to opposite outcomes of RR activity. Additionally, two important cancer drugs, gemcitabine and clofarabine are known to bind to higher-order RR oligomers. Thus, it is becoming increasingly clear that for an understanding of how RR is regulated in eukaryotes and targeted by cancer drugs, it is essential to elucidate the structure of higher-order oligomers formed by eukaryotic RRs. As higher-order holo-complexes may not be amenable to X-ray crystallography, we propose to use X-ray crystallography to determine the high-resolution structures of RR1 oligomers and single-particle electron microscopy (EM) to elucidate the organization of RR1 and RR2 in the holo-complexes.