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

No abstract

The 3D structure of the ternary complex, consisting of DNA ligase, the proliferating cell nuclear antigen (PCNA) clamp, and DNA, was investigated by single-particle analysis. This report presents the structural view, where the crescent-shaped DNA ligase with 3 distinct domains surrounds the central DNA duplex, encircled by the closed PCNA ring, thus forming a double-layer structure with dual contacts between the 2 proteins. The relative orientations of the DNA ligase domains, which remarkably differ from those of the known crystal structures, suggest that a large domain rearrangement occurs upon ternary complex formation. A second contact was found between the PCNA ring and the middle adenylation domain of the DNA ligase. Notably, the map revealed a substantial DNA tilt from the PCNA ring axis. This structure allows us to propose a switching mechanism for the replication factors operating on the PCNA ring.DNA replication ͉ electron microscopy ͉ single-particle analysis ͉ DNA sliding clamp ͉ protein-DNA complex D NA ligase plays essential roles in various DNA transactions, such as joining Okazaki fragments in DNA replication and nick-sealing at the final step of several DNA repair pathways, including nucleotide excision repair, base excision repair, and mismatch repair (1, 2). The enzyme catalyzes phosphodiester bond formation at the nicks generated within dsDNA, through the well-conserved 3-step reaction using either NAD ϩ or ATP as a cofactor (3). At the first step, the DNA ligase interacts with the nucleotide cofactor to form a covalent ligase-nucleoside monophosphate. At the second step, the bound AMP is transferred to the 5Ј terminus of the DNA, and finally, a phosphodiester bond is formed by a reaction between the 5Ј-DNAadenylate and the 3Ј-hydroxy group.To date, 3 crystal structures of ATP-dependent eukaryotictype DNA ligases have been reported. The structures of the 2 archaeal DNA ligases from Pyrococcus furiosus (PfuLig) and Sulfolobus solfataricus (SsoLig) were determined in the closed (4) and extended forms (5), respectively. The structure of the human DNA ligase 1 (hLigI) in complex with DNA (6) revealed that the enzyme entirely encircles the nicked DNA. All of these DNA ligases are in common composed of 3 domains, designated as the DNA binding domain (DBD), the adenylation domain (AdD), and the OB-fold domain (OBD), in the sequences from the N to C termini. Although the internal architectures of these domains are strikingly similar among the 3 DNA ligases, their relative domain orientations within each enzyme are quite different. Similar to many other replication factors, such as DNA polymerase and Flap endonuclease 1 (FEN1), DNA ligases exhibit the full activity by binding to proliferating cell nuclear antigen (PCNA). A small-angle X-ray scattering analysis revealed that the morphology of SsoLig in complex with PCNA coincides with the extended structure of SsoLig alone (5). However, the structure of the ternary Lig-PCNA-DNA complex remains unknown.PCNA interacts with various protein factors to contr...

DNA is constantly damaged by endogenous and environmental influences. Deaminated adenine (hypoxanthine) tends to pair with cytosine and leads to the A:T→G:C transition mutation during DNA replication. Endonuclease V (EndoV) hydrolyzes the second phosphodiester bond 3′ from deoxyinosine in the DNA strand, and was considered to be responsible for hypoxanthine excision repair. However, the downstream pathway after EndoV cleavage remained unclear. The activity to cleave the phosphodiester bond 5′ from deoxyinosine was detected in a Pyrococcus furiosus cell extract. The protein encoded by PF1551, obtained from the mass spectrometry analysis of the purified fraction, exhibited the corresponding cleavage activity. A putative homolog from Thermococcus kodakarensis (TK0887) showed the same activity. Further biochemical analyses revealed that the purified PF1551 and TK0887 proteins recognize uracil, xanthine and the AP site, in addition to hypoxanthine. We named this endonuclease Endonuclease Q (EndoQ), as it may be involved in damaged base repair in the Thermococcals of Archaea.

Trifluridine (FTD) is a key component of the novel oral antitumor drug TAS-102, which consists of FTD and a thymidine phosphorylase inhibitor. Like 5-fluoro-2 0 -deoxyuridine (FdUrd), a deoxynucleoside form of 5-fluorouracil metabolite, FTD is sequentially phosphorylated and not only inhibits thymidylate synthase activity, but is also incorporated into DNA. Although TAS-102 was effective for the treatment of refractory metastatic colorectal cancer in clinical trials, the mechanism of FTDinduced cytotoxicity is not completely understood. Here, we show that FTD as well as FdUrd induce transient phosphorylation of Chk1 at Ser345, and that this is followed by accumulation of p53 and p21 proteins in p53-proficient human cancer cell lines. In particular, FTD induced p53-dependent sustained arrest at G 2 phase, which was associated with a proteasome-dependent decrease in the Cyclin B1 protein level and the suppression of CCNB1 and CDK1 gene expression. In addition, a p53-dependent increase in p21 protein was associated with an FTD-induced decrease in Cyclin B1 protein. Although numerous ssDNA and dsDNA breaks were induced by FdUrd, few DNA strand breaks were detected in FTD-treated HCT-116 cells despite massive FTD misincorporation into genomic DNA, suggesting that the antiproliferative effect of FTD is not due to the induction of DNA strand breaks. These distinctive effects of FTD provide insights into the cellular mechanism underlying its antitumor effect and may explain the clinical efficacy of TAS-102.