Search

金沢大学がん研究所がん分子細胞制御, The assembly of RAD51 recombinase on DNA substrates at sites of breakage is essential for their repair by homologous recombination repair (HRR). The signaling pathway that triggers RAD51 assembly at damage sites to form subnuclear foci is unclear. Here, we provide evidence that c-ABL, a tyrosine kinase activated by DNA damage which phosphorylates RAD51 on Tyr-315, works at a previously unrecognized, proximal step to initiate RAD51 assembly. We first show that c-ABL associates with chromatin after DNA damage in a manner dependent on its kinase activity. Using RAD51 mutants that are unable to oligomerize to form a nucleoprotein filament, we separate RAD51 assembly on DNA to form foci into two steps: stable chromatin association followed by oligomerization. We show that phosphorylation on Tyr-315 by c-ABL is required for chromatin association of oligomerization-defective RAD51 mutants, but is insufficient to restore oligomerization. Our findings suggest a new model for the regulation of early steps of HRR. © 2009 Elsevier Inc. All rights reserved.

Experimental probing of a protein-folding energy land-scape can be challenging, and energy landscapes comprising multiple intermediates have not yet been defined. Here, we quasi-statically unfolded single molecules of staphylococcal nuclease by constant-rate mecahnical stretching with a feedback positioning system. Multiple discrete transition states were detected as force peaks, and only some of the multiple transition states emerged stochastically in each trial. This finding was confirmed by molecular dynamics simulations, and agreed with another result of the simulations which showed that individual trajectories took highly heterogeneous pathways. The presence of Ca2+ did not changethe location of the transition states, but changed the frequency of the emergence. Transition states emerged frequency of the emergence. Transition states emerged more frequently in stabilized domains. The simulations also confirmed this feature, and showed that the stabilized domains had rugged energy surfaces. The mean energy required per residue to disrupt secondary structures was a few times the thermal energy (1-3 kBT), which agreed with the stochastic feature. Thus, single-molecule quasi-static measurement has achieved notable success in detecting stochastic features of a huge number of possible conformations of a protein.

The DMC1 protein, a meiosis-specific DNA recombinase, catalyzes strand exchange between homologous chromosomes. In rice, two Dmc1 genes, Dmc1A and Dmc1B, have been reported. Although the Oryza sativa DMC1A protein has been partially characterized, however the biochemical properties of the DMC1B protein have not been defined. In the present study, we expressed the Oryza sativa DMC1A and DMC1B proteins in bacteria and purified them. The purified DMC1A and DMC1B proteins formed helical filaments along single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA), and promoted robust strand exchange between ssDNA and dsDNA over five thousand base pairs in the presence of RPA, as a co-factor. The DMC1A and DMC1B proteins also promoted strand exchange in the absence of RPA with long DNA substrates containing several thousand base pairs. In contrast, the human DMC1 protein strictly required RPA to promote strand exchange with these long DNA substrates. The strand-exchange activity of the Oryza sativa DMC1A protein was much higher than that of the DMC1B protein. Consistently, the DNA-binding activity of the DMC1A protein was higher than that of the DMC1B protein. These biochemical differences between the DMC1A and DMC1B proteins may provide important insight into their functional differences during meiosis in rice.

The Rad51 protein, which catalyzes homologous-pairing and strand-exchange reactions, is an essential enzyme for homologous recombinational repair (HRR) and meiotic homologous recombination in eukaryotes. In humans, the conventional Rad51 (HsRad51) protein has a Lys residue at position 313; however, the HsRad51-Q313 protein, in which the Lys313 residue is replaced by Gln, was reported as an isoform, probably corresponding to a polymorphic variant. In this study, we purified the HsRad51-K313 and HsRad51-Q313 isoforms and analyzed their biochemical activities in vitro. Compared to the conventional HsRad51-K313 protein, the HsRad51-Q313 protein exhibited significantly enhanced strand-exchange activity under conditions with Ca2+, although the difference was not observed without Ca2+. A double-stranded DNA (dsDNA) unwinding assay revealed that the HsRad51-Q313 protein clearly showed enhanced DNA unwinding activity, probably due to its enhanced filament-formation ability. Mutational analyses of the HsRad51-Lys313 residue revealed that positively charged residues (Lys and Arg), but not negatively charged, polar and hydrophobic residues (Glu, Gln and Met, respectively), at position 313 reduced the strand-exchange and DNA unwinding abilities of the HsRad51 protein. These results suggest that the electrostatic environment around position 313 is important for the regulation of the HsRad51 recombinase activity.

Na+チャネル遮断薬ピルジカイニド (Pil) がKv1.5チャネル蛋白を含む半減期の短い蛋白の増加作用を有することを見出したので, その機序をリドカイン (Lido) と比較して報告する.PilとLidoはプロテアソームで分解されるp53やIKK2蛋白を用量依存性に安定化し, このp53蛋白への作用は変異Mdmによりユビキチン・プロテアソーム系が遮断されると生じない, PilとLidoは用量依存性に20Sプロテアソーム活性を抑制するが, l-verapamilやE-4031にはこの作用は認められなかった.さらにPilはLidoに比較してより低濃度 (臨床使用濃度) でp53蛋白を安定化した.同様にPilとLidoはプロテアソームで分解されるKv1.5チャネル蛋白を安定化して細胞膜での/Kurを増加し, この作用は発現された培養COS7細胞Kv1.5チャネルのみならずラツト心房筋Kv1.5チャネルでも認められた, またKv1.5チャネル蛋白の安定化作用はPilがLidoより強い.その分子機序をin silicoドッキング試験から検討し, PilとLidoはともに20sプロテアソームを構成するβ5サブユニットの基質結合部位に作用して, その活性を抑制すると推測でき, PilがLidoに比較して結合性が高いことも再現できた, これらの結果からNa+チヤネル遮断薬Pilが臨床使用濃度でプロテアソーム活性を抑制し, Kv1.5チャネル蛋白を含む半減期の短い蛋白を増加させる作用を発揮することが明らかにされた.

To overcome the ensemble-averaging barrier, single-molecule experiments have been performed, but energy landscapes comprising multiple intermediates have not yet been defined. We performed mechanical unfolding of staphylococcal nuclease using intermoleculer force microscopy, modified AFM with high resolution and feedback control of the positioning. The force dropped vertically just after its peak, and multiple transition states were detected as force peaks. The multiple and stochastic intermediates found in the present study provide new important information on protein folding.

The RAD51 protein is a central player in homologous recombinational repair. The RAD51B protein is one of five RAD51 paralogs that function in the homologous recombinational repair pathway in higher eukaryotes. In the present study, we found that the human EVL (Ena/Vasp-like) protein, which is suggested to be involved in actin-remodeling processes, unexpectedly binds to the RAD51 and RAD51B proteins and stimulates the RAD51-mediated homologous pairing and strand exchange. The EVL knockdown cells impaired RAD51 assembly onto damaged DNA after ionizing radiation or mitomycin C treatment. The EVL protein alone promotes single-stranded DNA annealing, and the recombination activities of the EVL protein are further enhanced by the RAD51B protein. The expression of the EVL protein is not ubiquitous, but it is significantly expressed in breast cancer-derived MCF7 cells. These results suggest that the EVL protein is a novel recombination factor that may be required for repairing specific DNA lesions, and that may cause tumor malignancy by its inappropriate expression.

It is difficult to determine the structural stability of the individual subunits or protomers of many proteins in the cell that exist in an oligomeric or complexed state. In this study, we used single-molecule force spectroscopy on seven subunits of covalently linked cochaperonin GroES (ESC7) to evaluate the structural stability of the subunit. A modified form of ESC7 was immobilized on a mica surface. The force-extension profile obtained from the mechanical unfolding of this ESC7 showed a distinctive sawtooth pattern that is typical for multimodular proteins. When analyzed according to the worm-like chain model, the contour lengths calculated from the peaks in the profile suggested that linked-GroES subunits unfold in distinct steps after the oligomeric ring structure of ESC7 is disrupted. The evidence that structured subunits of ESC7 withstand external force to some extent even after the perturbation of the oligomeric ring structure suggests that a stable monomeric intermediate is an important component of the equilibrium unfolding reaction of GroES.

The Rad51 protein, which catalyzes homologous-pairing and strand-exchange reactions, is an essential enzyme for homologous recombinational repair (HRR) and meiotic homologous recombination in eukaryotes. In humans, the conventional Rad51 (HsRad51) protein has a Lys residue at position 313; however, the HsRad51-Q313 protein, in which the Lys313 residue is replaced by Gln, was reported as an isoform, probably corresponding to a polymorphic variant. In this study, we purified the HsRad51-K313 and HsRad51-Q313 isoforms and analyzed their biochemical activities in vitro. Compared to the conventional HsRad51-K313 protein, the HsRad51-Q313 protein exhibited significantly enhanced strand-exchange activity under conditions with Ca(2+), although the difference was not observed without Ca(2+). A double-stranded DNA (dsDNA) unwinding assay revealed that the HsRad51-Q313 protein clearly showed enhanced DNA unwinding activity, probably due to its enhanced filament-formation ability. Mutational analyses of the HsRad51-Lys313 residue revealed that positively charged residues (Lys and Arg), but not negatively charged, polar and hydrophobic residues (Glu, Gln and Met, respectively), at position 313 reduced the strand-exchange and DNA unwinding abilities of the HsRad51 protein. These results suggest that the electrostatic environment around position 313 is important for the regulation of the HsRad51 recombinase activity.

International audience; The M200V polymorphism of the human DMC1 protein, which is an essential, meiosis-specific DNA recombinase, was found in an infertile patient, raising the question of whether this homozygous human DMC1-M200V polymorphism may cause infertility by affecting the function of the human DMC1 protein. In the present study, we determined the crystal structure of the human DMC1-M200V variant in the octameric-ring form. Biochemical analyses revealed that the human DMC1-M200V variant had reduced stability, and was moderately defective in catalyzing in vitro recombination reactions. The corresponding M194V mutation introduced in the Schizosaccharomyces pombe dmc1 gene caused a significant decrease in the meiotic homologous recombination frequency. Together, these structural, biochemical and genetic results provide extensive evidence that the human DMC1-M200V mutation impairs its function, supporting the previous interpretation that this single-nucleotide polymorphism is a source of human infertility.

The human Rad51 protein (HsRad51) catalyzes homologous pairing and strand exchange between single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) during recombinational repair of double-stranded DNA breaks. An HsRad51 mutation that results in the substitution of Gln for Arg150 (R150Q) was found in bilateral breast cancer patients; however, the consequences of this R150Q mutation have not been elucidated. To determine how this HsRad51(R150Q) mutation affects HsRad51 function, in the present study, we purified the HsRad51(R150Q) mutant. The purified HsRad51(R150Q) was completely proficient in the ATP-hydrolyzing activity. A gel filtration analysis revealed that HsRad51(R150Q) also retained the polymer formation ability. In contrast, the ssDNA- and dsDNA-binding abilities of HsRad51(R150Q) were clearly reduced, as compared to those of HsRad51. These differences in the DNA-binding properties between HsRad51(R150Q) and HsRad51 may be important to account for the tumorigenesis in breast cancer patients with the HsRad51(R150Q) mutation.

The human Rad51 protein, a eukaryotic ortholog of the bacterial RecA protein, is a key enzyme that functions in homologous recombination and recombinational repair of double strand breaks. The Rad51 protein contains two flexible loops, L1 and L2, which are proposed to be sites for DNA binding, based on a structural comparison with RecA. In the present study, we performed mutational and fluorescent spectroscopic analyses on the L1 and L2 loops to examine their role in DNA binding. Gel retardation and DNA-dependent ATP hydrolysis measurements revealed that the substitution of the tyrosine residue at position 232 (Tyr232) within the L1 loop with alanine, a short side chain amino acid, significantly decreased the DNA-binding ability of human Rad51, without affecting the protein folding or the salt-induced, DNA-independent ATP hydrolysis. Even the conservative replacement with tryptophan affected the DNA binding, indicating that Tyr232 is involved in DNA binding. The importance of the L1 loop was confirmed by the fluorescence change of a tryptophan residue, replacing the Asp231, Ser233, or Gly236 residue, upon DNA binding. The alanine replacement of phenylalanine at position 279 (Phe279) within the L2 loop did not affect the DNA-binding ability of human Rad51, unlike the Phe203 mutation of the RecA L2 loop. The Phe279 side chain may not be directly involved in the interaction with DNA. However, the fluorescence intensity of the tryptophan replacing the Rad51-Phe279 residue was strongly reduced upon DNA binding, indicating that the L2 loop is also close to the DNA-binding site.