title: The conserved Lys-95 charged residue cluster is critical for the homodimerization and enzyme activity of human ribonucleotide reductase small subunit M2.
abstract: Ribonucleotide reductase (RR) catalyzes the reduction of ribonucleotides to deoxyribonucleotides for DNA synthesis. Human RR small subunit M2 exists in a homodimer form. However, the importance of the dimer form to the enzyme and the related mechanism remain unclear. In this study, we tried to identify the interfacial residues that may mediate the assembly of M2 homodimer by computational alanine scanning based on the x-ray crystal structure. Co-immunoprecipitation, size exclusion chromatography, and RR activity assays showed that the K95E mutation in M2 resulted in dimer disassembly and enzyme activity inhibition. In comparison, the charge-exchanging double mutation of K95E and E98K recovered the dimerization and activity. Structural comparisons suggested that a conserved cluster of charged residues, including Lys-95, Glu-98, Glu-105, and Glu-174, at the interface may function as an ionic lock for M2 homodimer. Although the measurements of the radical and iron contents showed that the monomer (the K95E mutant) was capable of generating the diiron and tyrosyl radical cofactor, co-immunoprecipitation and competitive enzyme inhibition assays indicated that the disassembly of M2 dimer reduced its interaction with the large subunit M1. In addition, the immunofluorescent and fusion protein-fluorescent imaging analyses showed that the dissociation of M2 dimer altered its subcellular localization. Finally, the transfection of the wild-type M2 but not the K95E mutant rescued the G1/S phase cell cycle arrest and cell growth inhibition caused by the siRNA knockdown of M2. Thus, the conserved Lys-95 charged residue cluster is critical for human RR M2 homodimerization, which is indispensable to constitute an active holoenzyme and function in cells.
authors.affiliation: From the Department of Pathology and Pathophysiology, Zhejiang University School of Medicine, Hangzhou 310058, China.
authors.forename: Xinhuan
authors.initials: X
authors.lastname: Chen
authors.forename: Zhijian
authors.initials: Z
authors.lastname: Xu
authors.forename: Lingna
authors.initials: L
authors.lastname: Zhang
authors.forename: Hongchuan
authors.initials: H
authors.lastname: Liu
authors.forename: Xia
authors.initials: X
authors.lastname: Liu
authors.forename: Meng
authors.initials: M
authors.lastname: Lou
authors.forename: Lijun
authors.initials: L
authors.lastname: Zhu
authors.forename: Bingding
authors.initials: B
authors.lastname: Huang
authors.forename: Cai-Guang
authors.initials: CG
authors.lastname: Yang
authors.forename: Weiliang
authors.initials: W
authors.lastname: Zhu
authors.forename: Jimin
authors.initials: J
authors.lastname: Shao
mesh_terms: D019943:Amino Acid Substitution; D055162:Biocatalysis; D049109:Cell Proliferation; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D059585:G1 Phase Cell Cycle Checkpoints / Q000235:genetics; D018698:Glutamic Acid / Q000235:genetics / Q000378:metabolism*; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D057809:HEK293 Cells; D006367:HeLa Cells; D006801:Humans; D015151:Immunoblotting; D008239:Lysine / Q000235:genetics / Q000378:metabolism*; D018613:Microscopy, Confocal; D008958:Models, Molecular; D009154:Mutation; D055503:Protein Multimerization; D034622:RNA Interference; D012262:Ribonucleoside Diphosphate Reductase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D049452:Green Fluorescent Proteins; D018698:Glutamic Acid; C093713:ribonucleotide reductase M2; D012262:Ribonucleoside Diphosphate Reductase; D008239:Lysine
keywords: Enzyme Catalysis; Enzyme Mechanisms; Human Ribonucleotide Reductase Small Subunit M2; Protein Crystallization; Protein Structure; Site-directed Mutagenesis