Background: Evidence regarding the primary prevention of coronary artery disease events by low-density lipoprotein cholesterol (LDL-C) lowering therapy in older individuals, aged ≥75 years, is insufficient. This trial tested whether LDL-C–lowering therapy with ezetimibe is useful for the primary prevention of cardiovascular events in older patients. Methods: This multicenter, prospective, randomized, open-label, blinded end-point evaluation conducted at 363 medical institutions in Japan examined the preventive efficacy of ezetimibe for patients aged ≥75 years, with elevated LDL-C without history of coronary artery disease. Patients, who all received dietary counseling, were randomly assigned (1:1) to receive ezetimibe (10 mg once daily) versus usual care with randomization stratified by site, age, sex, and baseline LDL-C. The primary outcome was a composite of sudden cardiac death, myocardial infarction, coronary revascularization, or stroke. Results: Overall, 3796 patients were enrolled between May 2009 and December 2014, and 1898 each were randomly assigned to ezetimibe versus control. Median follow-up was 4.1 years. After exclusion of 182 ezetimibe patients and 203 control patients because of lack of appropriate informed consent and other protocol violations, 1716 (90.4%) and 1695 (89.3%) patients were included in the primary analysis, respectively. Ezetimibe reduced the incidence of the primary outcome (hazard ratio [HR], 0.66; 95% CI, 0.50–0.86; P =0.002). Regarding the secondary outcomes, the incidences of composite cardiac events (HR, 0.60; 95% CI, 0.37–0.98; P =0.039) and coronary revascularization (HR, 0.38; 95% CI, 0.18–0.79; P =0.007) were lower in the ezetimibe group than in the control group; however, there was no difference in the incidence of stroke, all-cause mortality, or adverse events between trial groups. Conclusions: LDL-C–lowering therapy with ezetimibe prevented cardiovascular events, suggesting the importance of LDL-C lowering for primary prevention in individuals aged ≥75 years with elevated LDL-C. Given the open-label nature of the trial, its premature termination and issues with follow-up, the magnitude of benefit observed should be interpreted with caution. Clinical Registration: URL: https://www.umin.ac.jp . Unique identifier: UMIN000001988.

The three-dimensional structure of NADH-cytochrome b5 reductase from pig liver microsomes has been determined at 2.4 A resolution by X-ray crystallography. The molecular structure reveals two domains, the FAD binding domain and the NADH domain. A large cleft lies between these two domains and contains the binding site for the FAD prosthetic group. The backbone structure of the FAD binding domain has a great similarity to that of ferredoxin-NADP+ reductase [Karplus, P. A., Daniels, M. J., & Herriott, J. R. (1991) Science 251, 60-65], in spite of the relatively low sequence homology (about 15%) between the two enzymes. On the other hand, the structure of the NADH domain has several structural differences from that of the NADP+ domain of ferredoxin-NADP+ reductase. The size of the cleft between the two domains is larger in NADH-cytochrome b5 reductase than in ferredoxin-NADP+ reductase, which may be responsible for the observed difference in the nucleotide accessibility in the two enzymes.

DNA replication in archaea and eukaryotes is executed by family B DNA polymerases, which exhibit full activity when complexed with the DNA clamp, proliferating cell nuclear antigen (PCNA). This replication enzyme consists of the polymerase and exonuclease moieties responsible for DNA synthesis and editing (proofreading), respectively. Because of the editing activity, this enzyme ensures the high fidelity of DNA replication. However, it remains unclear how the PCNA-complexed enzyme temporally switches between the polymerizing and editing modes. Here, we present the threedimensional structure of the Pyrococcus furiosus DNA polymerase B-PCNA-DNA ternary complex, which is the core component of the replisome, determined by single particle electron microscopy of negatively stained samples. This structural view, representing the complex in the editing mode, revealed the whole domain configuration of the trimeric PCNA ring and the DNA polymerase, including protein-protein and protein-DNA contacts. Notably, besides the authentic DNA polymerase-PCNA interaction through a PCNAinteracting protein (PIP) box, a novel contact was found between DNA polymerase and the PCNA subunit adjacent to that with the PIP contact. This contact appears to be responsible for the configuration of the complex specific for the editing mode. The DNA was located almost at the center of PCNA and exhibited a substantial and particular tilt angle against the PCNA ring plane. The obtained molecular architecture of the complex, including the new contact found in this work, provides clearer insights into the switching mechanism between the two distinct modes, thus highlighting the functional significance of PCNA in the replication process.fidelity control | protein-DNA complex | replication fork | single particle analysis | structural bioinformatics

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Proliferating cell nuclear antigen (PCNA) is responsible for the processivity of DNA polymerase. We determined the crystal structure of Pyrococcus furiosus DNA polymerase (PfuPol) complexed with the cognate monomeric PCNA, which allowed us to construct a convincing model of the polymerase-PCNA ring interaction, with unprecedented configurations of the two molecules. Electron microscopic analyses indicated that this complex structure exists in solution. Our structural study revealed that an interaction occurs between a stretched loop of PCNA and the PfuPol Thumb domain, in addition to the authentic PCNA-polymerase recognition site (PIP box). Comparisons of the present structure with the previously reported structures of polymerases complexed with DNA, suggested that the second interaction plays a crucial role in switching between the polymerase and exonuclease modes, by inducing a PCNA-polymerase complex configuration that favors synthesis over editing. This putative mechanism for fidelity control of replicative DNA polymerases is supported by experiments, in which mutations at the second interaction site caused enhancements in the exonuclease activity in the presence of PCNA.DNA clamp ͉ DNA replication ͉ electron microscopy ͉ fidelity control ͉ protein crystallography D NA replication is a highly coordinated process, in which DNA polymerase and the DNA sliding clamp play major roles in ensuring accurate genome duplication. Most replicative DNA polymerases consist of two independent functional components: polymerase and exonuclease moieties, which execute DNA synthesis and editing, respectively (1, 2). Since these two active sites are apart from each other, the DNA substrate cannot access both sites at the same time. These two reactions are performed alternately, and thus require regulation by temporally different switching, which was previously proposed from a comparison of the crystal structures of RB69 DNA polymerases complexed with dsDNA in the polymerase and exonuclease modes (3, 4).The DNA clamp, which forms a dimer or trimer, depending on the domain of life, retains DNA polymerase on the template DNA strand for processive DNA synthesis. DNA clamps, which are called PCNA in Archaea and Eukarya, and the ␤ subunit in Bacteria, form similar architectures with a central hole. Thus, they encircle the substrate DNA, thereby enabling consecutive DNA synthesis (5).PCNA-interacting proteins contain a small conserved motif, the PIP (PCNA-interacting protein) box, which binds to a common site on PCNA (6, 7). A model building study between the RB69 DNA polymerase-editing complex and PCNA revealed the consistent configuration of each component (3). The structure of the complex between the ''little finger'' (LF) domain of the translesional Y-family DNA polymerase Pol IV and the ␤-clamp from Escherichia coli showed that the bound polymerase domain lies at the outer rim of the clamp ring (8).We reported previously that the PCNA from the hyperthermophilic archaeon, Pyrococcus furiosus, bound to the PIP box of the family B D...

DNA ligase is an essential enzyme for all organisms and catalyzes a nick-joining reaction in the final step of the DNA replication, repair, and recombination processes. Herein, we show the physical and functional interaction between DNA ligase and proliferating cell nuclear antigen (PCNA) from the hyperthermophilic Euryarchaea Pyrococcus furiosus. The stimulatory effect of P. furiosus PCNA on the enzyme activity of P. furiosus DNA ligase was observed not at low ionic strength, but at a high salt concentration, at which a DNA ligase alone cannot bind to a nicked DNA substrate. On the basis of mutational analyses, we identified the amino acid residues that are critical for PCNA binding in a loop structure located in the N-terminal DNA-binding domain of P. furiosus DNA ligase. We propose that the pentapeptide motif QKSFF is involved in the PCNA-interacting motifs, in which Gln and the first Phe are especially important for stable binding with PCNA.

Porcine NADH-cytochrome b5 reductase catalytic domain (Pb5R) has the RXY(T/S)+(T/S) flavin-binding motif that is highly conserved among the structurally related family of flavoprotein reductases. Mutations were introduced that alter the Arg(63), Tyr(65), and Ser(99) residues within this motif. The mutation of Tyr(65) to either alanine or phenylalanine destabilized the protein, produced an accelerated release of FAD in the presence of 1.5 M guanidine hydrochloride, and decreased the k(cat) values of the enzyme. These results indicate that Tyr(65) contributes to the stability of the protein and is important in the electron transfer from NADH to FAD. The mutation of Ser(99) to either alanine or valine, and of Arg(63) to either alanine or glutamine increased both the K(m) values for NADH (K(m)(NADH)) and the dissociation constant for NAD(+) (K(d)(NAD+)). However, the mutation of Ser(99) to threonine and of Arg(63) to lysine had very little effect on the K(m)(NADH) and K(d)(NAD+) values, and resulted in small changes in the absorption and circular dichroism spectra. These results suggest that the hydroxyl group of Ser(99) and the positive charge of Arg(63) contribute to the maintenance of the properties of FAD and to the effective binding of Pb5R to both NADH and NAD(+). In addition, the mutation of Arg(63) to either alanine or glutamine increased the apparent K(m) values for porcine cytochrome b5 (Pb5), while changing Arg(63) to lysine did not. The positive charge of Arg(63) may regulate the electron transfer through the electrostatic interaction with Pb5. These results substantiate the important role of the flavin-binding motif in Pb5R.

DNA ligases catalyze the joining of strand breaks in duplex DNA. The DNA ligase of Pyrococcus furiosus (PfuLig), which architecturally resembles the human DNA ligase I (hLigI), comprises an N-terminal DNA-binding domain, a middle adenylylation domain, and a C-terminal oligonucleotide-binding (OB)-fold domain. Here we addressed the C-terminal helix in the OB-fold domain of PfuLig by mutational analysis. The crystal structure of PfuLig revealed that this helix stabilizes a closed conformation of the enzyme by forming several ionic interactions with the adenylylation domain. The C-terminal helix is oriented differently in hLigI when DNA is bound; this suggested that disruption of its interaction with the adenylylation domain might facilitate the binding of DNA substrates. We indeed identified one of its residues, Asp540, as being critical for ligation efficiency. The D540R mutation improved the overall ligation activity relative to the wild-type enzyme, and at lower temperatures; this is relevant to applications such as ligation amplification reactions. Physical and biochemical analyses indicated that the improved ligation activity of the D540R variant arises from effects on the ligase adenylylation step and on substrate DNA binding in particular.