Antiviral nucleoside analogs have been developed to inhibit the enzymatic activities of the hepat... more Antiviral nucleoside analogs have been developed to inhibit the enzymatic activities of the hepatitis B virus (HBV) polymerase, thereby preventing the replication and production of HBV. However, the usage of these analogs can be limited by drug toxicity because the 5′-triphosphates of these nucleoside analogs (nucleotide analogs) are potential substrates for human DNA polymerases to incorporate into host DNA. Although they are poor substrates for human replicative DNA polymerases, it remains to be established whether these nucleotide analogs are substrates for the recently discovered human X-and Y-family DNA polymerases. Using presteady state kinetic techniques, we have measured the substrate specificity values for human DNA polymerases β, λ, η, ι, κ, and Rev1 incorporating the active forms of the following anti-HBV nucleoside analogs approved for clinical use: adefovir, tenofovir, lamivudine, telbivudine, and entecavir. Compared to the incorporation of a natural nucleotide, most of the nucleotide analogs were incorporated less efficiently (2 to >122,000) by the six human DNA polymerases. In addition, the potential for entecavir and telbivudine, two drugs which possess a 3′-hydroxyl, to become embedded into human DNA was examined by primer extension and DNA ligation assays. These results suggested that telbivudine functions as a chain terminator while entecavir was efficiently extended by the six enzymes and was a substrate for human DNA ligase I. Our findings suggested that incorporation of anti-HBV nucleotide analogs catalyzed by human X-and Y-family polymerases may contribute to clinical toxicity.
DNA polymerase λ (Pol λ) is a novel X-family DNA polymerase that shares 34% sequence identity wit... more DNA polymerase λ (Pol λ) is a novel X-family DNA polymerase that shares 34% sequence identity with DNA polymerase β (Pol β). Pre-steady state kinetic studies have shown that the Pol λ•DNA complex binds both correct and incorrect nucleotides 130-fold tighter on average than the Pol β•DNA complex, although, the base substitution fidelity of both polymerases is 10 -4 to 10 -5 . To better understand Pol λ's tight nucleotide binding affinity, we created single-and doublesubstitution mutants of Pol λ to disrupt interactions between active site residues and an incoming nucleotide or a template base. Single-turnover kinetic assays showed that Pol λ binds to an incoming nucleotide via cooperative interactions with active site residues (R386, R420, K422, Y505, F506, A510, and R514). Disrupting protein interactions with an incoming correct or incorrect nucleotide impacted binding with each of the common structural moieties in the following order: triphosphate ≫ base > ribose. In addition, the loss of Watson-Crick hydrogen bonding between the nucleotide and template base led to a moderate increase in the K d . The fidelity of Pol λ was maintained predominantly by a single residue, R517, which has minor groove interactions with the DNA template.
During DNA synthesis, most DNA polymerases and reverse transcriptases select against ribonucleoti... more During DNA synthesis, most DNA polymerases and reverse transcriptases select against ribonucleotides via a steric clash between the ribose 2′-hydroxyl group and the bulky side chain of an active site residue. Here, we demonstrated that human DNA polymerase λ used a novel sugar selection mechanism to discriminate against ribonucleotides, whereby the ribose 2′-hydroxyl group was excluded mostly by a backbone segment and slightly by the side chain of Y505. Such a steric clash was further demonstrated to be dependent on the size and orientation of the substituent covalently attached at the ribonucleotide C2′ position.
DNA polymerase λ (Pol λ) is a novel X-family DNA polymerase that shares 34% sequence identity wit... more DNA polymerase λ (Pol λ) is a novel X-family DNA polymerase that shares 34% sequence identity with DNA polymerase β (Pol β). Pre-steady state kinetic studies have shown that the Pol λ•DNA complex binds both correct and incorrect nucleotides 130-fold tighter on average than the Pol β•DNA complex, although, the base substitution fidelity of both polymerases is 10 -4 to 10 -5 . To better understand Pol λ's tight nucleotide binding affinity, we created single-and doublesubstitution mutants of Pol λ to disrupt interactions between active site residues and an incoming nucleotide or a template base. Single-turnover kinetic assays showed that Pol λ binds to an incoming nucleotide via cooperative interactions with active site residues (R386, R420, K422, Y505, F506, A510, and R514). Disrupting protein interactions with an incoming correct or incorrect nucleotide impacted binding with each of the common structural moieties in the following order: triphosphate ≫ base > ribose. In addition, the loss of Watson-Crick hydrogen bonding between the nucleotide and template base led to a moderate increase in the K d . The fidelity of Pol λ was maintained predominantly by a single residue, R517, which has minor groove interactions with the DNA template.
1-Nitropyrene, the most abundant nitro polycyclic aromatic hydrocarbon in diesel emissions, has b... more 1-Nitropyrene, the most abundant nitro polycyclic aromatic hydrocarbon in diesel emissions, has been found to react with DNA to form predominantly N-(deoxyguanosin-8-yl)-1-aminopyrene (dG AP ). This bulky adduct has been shown to induce genetic mutations, which may implicate Y-family DNA polymerases in its bypass in vivo. To establish a kinetic mechanism for the bypass of such a prototype single-base lesion, we employed pre-steady-state kinetic methods to investigate individual nucleotide incorporations upstream, opposite, and downstream from a site-specifically placed dG AP lesion catalyzed by Sulfolobus solfataricus DNA polymerase IV (Dpo4), a model Y-family DNA polymerase. Dpo4 was able to bypass dG AP but paused strongly at two sites: opposite the lesion and immediately downstream from the lesion. Both nucleotide incorporation efficiency and fidelity decreased significantly at the pause sites, especially during extension of the bypass product. Interestingly, a 4-fold tighter binding affinity of damaged DNA to Dpo4 promoted catalysis through putative interactions between the active site residues of Dpo4 and 1-aminopyrene moiety at the first pause site. In the presence of a DNA trap, the kinetics of nucleotide incorporation at these sites was biphasic in which a small, fast phase preceded a larger, slow phase. In contrast, only a large, fast phase was observed during nucleotide incorporation at non-pause sites. Our kinetic studies support a general kinetic mechanism for lesion bypass catalyzed by numerous DNA polymerases. Environmental pollutants have been shown to impact human health at the molecular level. One detrimental route is the mod-ification of genomic DNA and nucleotides (1). If DNA lesions are not recognized and removed by the cellular DNA repair machinery, they will stall replicative DNA polymerases (2-8). To rescue DNA replication, cells employ lesion bypass DNA polymerases to traverse unrepaired lesions. Most of these enzymes belong to the Y-family of DNA polymerases. The Y-family enzymes possess relatively flexible and solvent-accessible active sites to accommodate bulky DNA lesions (9, 10). However, Y-family DNA polymerases catalyze DNA synthesis over undamaged DNA with low fidelity and poor processivity (6, 10 -12). The Y-family DNA polymerases have been identified in all three domains of life, e.g. four in humans (DNA polymerases , , , and Rev1), two in Escherichia coli (DNA polymerases IV and V), and one in Sulfolobus solfataricus (Dpo4). Because Dpo4 can be expressed in E. coli and purified with a high yield, it has been extensively studied in vitro as a prototype Y-family enzyme. Dpo4 catalyzes DNA synthesis on an undamaged DNA template with a fidelity of one error per 1,000 -10,000 nucleotide incorporations based on presteady-state kinetic analysis from 37 to 56 °C (13-15). Dpo4 is capable of bypassing a myriad of DNA lesions including apurinic/apyrimidinic (abasic) sites (16 -19), 8-oxo-7,8-dihydro-2Ј-deoxyguanosine (20, 21), 1,N 2 -etheno(⑀)guanosine ( ), cis-syn thymine-thymine dimer (23-25), cisplatin-induced 1,2-intrastrand cross-links with adjacent deoxyguanosines (cisplatin-d(GpG) adducts) , benzo[a]pyrene diol epoxide (BPDE) 5 on deoxyguanosine (BPDE-dG) or deoxyadenosine (BPDE-dA) (27, 28), and N-2-acetyl-aminofluorene (AAF) on deoxyguanosine (AAF-dG) (24). So far there are no comprehensive in vitro studies of the bypass of 1-nitropyrene (1-NP)-induced DNA adducts catalyzed by a Y-family DNA polymerase. 1-NP, one of the most abundant polycyclic aromatic hydrocarbons, is a product of incomplete diesel and gasoline combustion . There are two known pathways by which 1-NP is metabolized: nitro reduction (Scheme 1) and C-hydroxylation. When an aromatic ring of 1-NP is oxidized into non-DNA-reactive metabolites by
Sulfolobus solfataricusDNA Polymerase IV (Dpo4), a prototype Y-family DNA polymerase, has been we... more Sulfolobus solfataricusDNA Polymerase IV (Dpo4), a prototype Y-family DNA polymerase, has been well characterized biochemically and biophysically at 37 °C or lower temperatures. However, the physiological temperature of the hyperthermophile S. solfataricus is approximately 80 °C. With such a large discrepancy in temperature, the in vivo relevance of these in vitro studies of Dpo4 has been questioned. Here, we employed circular dichroism spectroscopy and fluorescence-based thermal scanning to investigate the secondary structural changes of Dpo4 over a temperature range from 26 to 119 °C. Dpo4 was shown to display a high melting temperature characteristic of hyperthermophiles. Unexpectedly, the Little Finger domain of Dpo4, which is only found in the Y-family DNA polymerases, was shown to be more thermostable than the polymerase core. More interestingly, Dpo4 exhibited a three-state cooperative unfolding profile with an unfolding intermediate. The linker region between the Little Finger and Thumb domains of Dpo4 was found to be a source of structural instability. Through site-directed mutagenesis, the interactions between the residues in the linker region and the Palm domain were identified to play a critical role in the formation of the unfolding intermediate. Notably, the secondary structure of Dpo4 was not altered when the temperature was increased from 26 to 87.5 °C. Thus, in addition to providing structural insights into the thermal stability and an unfolding intermediate of Dpo4, our work also validated the relevance of the in vitro studies of Dpo4 performed at temperatures significantly lower than 80 °C.
Rev1, a Y-family DNA polymerase, contributes to spontaneous and DNA damage-induced mutagenic even... more Rev1, a Y-family DNA polymerase, contributes to spontaneous and DNA damage-induced mutagenic events. In this paper, we have employed pre-steady state kinetic methodology to establish a kinetic basis for nucleotide selection by human Rev1, a unique nucleotidyl transferase that uses a protein template-directed mechanism to preferentially instruct dCTP incorporation. This work demonstrated that the high incorporation efficiency of dCTP is dependent on both substrates: an incoming dCTP and a templating base dG. The extremely low base substitution fidelity of human Rev1 (10 0 to 10 -5 ) was due to the preferred misincorporation of dCTP with templating bases dA, dT, and dC over correct dNTPs. Using non-natural nucleotide analogs, we showed that hydrogen bonding interactions between residue R357 of human Rev1 and an incoming dNTP are not essential for DNA synthesis. Lastly, human Rev1 discriminates between ribonucleotides and deoxyribonucleotides mainly by reducing the rate of incorporation, and the sugar selectivity of human Rev1 is sensitive to both the size and orientation of the 2′-substituent of a ribonucleotide. The human genome encodes at least 16 DNA polymerases (Pol) that are involved in replicating and maintaining the integrity of genomic DNA. Human DNA polymerases are classified into four families: A, B, X, and Y. Y-family DNA polymerases are involved in DNA damage tolerance pathways, whereby a Y-family enzyme rescues stalled DNA replication at sites of DNA damage. Humans have four known Y-family members: Pol η, Pol ι, Pol κ, and Rev1. Rev1 is found in the genome of all eukaryotes (1) and is capable of functioning in both catalytic and structural roles. Composed of 1,251 amino acids (2), human Rev1 (hRev1) is organized into a central catalytic domain that is flanked by an Nterminal BRCT domain and a C-terminus with two ubiquitin-binding motifs and a domain for polymerase interactions (3). As a scaffold protein, Rev1 interacts with proliferating cell nuclear antigen (PCNA) (4-7), ubiquitinated proteins and DNA polymerases η, κ, ι,. These findings support a model, whereby Rev1 is involved in polymerase
Nucleoside reverse transcriptase inhibitors (NRTIs) are an important class of antiviral drugs use... more Nucleoside reverse transcriptase inhibitors (NRTIs) are an important class of antiviral drugs used to manage infections by human immunodeficiency virus, which causes AIDS. Unfortunately, these drugs cause unwanted side effects, and the molecular basis of NRTI toxicity is not fully understood. Putative routes of NRTI toxicity include the inhibition of human nuclear and mitochondrial DNA polymerases. A strong correlation between mitochondrial toxicity and NRTI incorporation catalyzed by human mitochondrial DNA polymerase has been established both in vitro and in vivo . However, it remains to be determined whether NRTIs are substrates for the recently discovered human X- and Y-family DNA polymerases, which participate in DNA repair and DNA lesion bypass in vivo . Using pre-steady-state kinetic techniques, we measured the substrate specificity constants for human DNA polymerases β, λ, η, ι, κ, and Rev1 incorporating the active, 5′-phosphorylated forms of tenofovir, lamivudine, emtricita...
Antiviral nucleoside analogs have been developed to inhibit the enzymatic activities of the hepat... more Antiviral nucleoside analogs have been developed to inhibit the enzymatic activities of the hepatitis B virus (HBV) polymerase, thereby preventing the replication and production of HBV. However, the usage of these analogs can be limited by drug toxicity because the 5′-triphosphates of these nucleoside analogs (nucleotide analogs) are potential substrates for human DNA polymerases to incorporate into host DNA. Although they are poor substrates for human replicative DNA polymerases, it remains to be established whether these nucleotide analogs are substrates for the recently discovered human X-and Y-family DNA polymerases. Using presteady state kinetic techniques, we have measured the substrate specificity values for human DNA polymerases β, λ, η, ι, κ, and Rev1 incorporating the active forms of the following anti-HBV nucleoside analogs approved for clinical use: adefovir, tenofovir, lamivudine, telbivudine, and entecavir. Compared to the incorporation of a natural nucleotide, most of the nucleotide analogs were incorporated less efficiently (2 to >122,000) by the six human DNA polymerases. In addition, the potential for entecavir and telbivudine, two drugs which possess a 3′-hydroxyl, to become embedded into human DNA was examined by primer extension and DNA ligation assays. These results suggested that telbivudine functions as a chain terminator while entecavir was efficiently extended by the six enzymes and was a substrate for human DNA ligase I. Our findings suggested that incorporation of anti-HBV nucleotide analogs catalyzed by human X-and Y-family polymerases may contribute to clinical toxicity.
Considering that all natural nucleotides (D-dNTPs) and the building blocks (D-dNMPs) of DNA chain... more Considering that all natural nucleotides (D-dNTPs) and the building blocks (D-dNMPs) of DNA chains possess D-stereochemistry, DNA polymerases and reverse transcriptases (RTs) likely possess strongD-stereoselectivity by preferably binding and incorporating D-dNTPs over unnatural L-dNTPs during DNA synthesis. Surprisingly, a structural basis for the discrimination against L-dNTPs by DNA polymerases or RTs has not been established although L-deoxycytidine analogs (lamivudine and emtricitabine) and L-thymidine (telbivudine) have been widely used as antiviral drugs for years. Here we report seven high-resolution ternary crystal structures of a prototype Y-family DNA polymerase, DNA, and D-dCTP, D-dCDP, L-dCDP, or the diphosphates and triphosphates of lamivudine and emtricitabine. These structures reveal that relative to D-dCTP, each of these L-nucleotides has its sugar ring rotated by 180° with an unusual O4'-endo sugar puckering and exhibits multiple triphosphate-binding conformatio...
Sulfolobus solfataricus DNA polymerase IV (Dpo4), a prototype Y-family DNA polymerase, contains a... more Sulfolobus solfataricus DNA polymerase IV (Dpo4), a prototype Y-family DNA polymerase, contains a unique little finger domain besides a catalytic core. Here, we report the chemical shift assignments for the backbone nitrogens, a and b carbons, and amide protons of the little finger domain of Dpo4. This work and our published backbone assignment for the catalytic core provide the basis for investigating the conformational dynamics of Dpo4 during catalysis using solution NMR spectroscopy.
Sulfolobus solfataricus DNA polymerase IV (Dpo4), a model Y-family DNA polymerase, bypasses DNA l... more Sulfolobus solfataricus DNA polymerase IV (Dpo4), a model Y-family DNA polymerase, bypasses DNA lesions. Here, we report the assignments for the backbone nitrogen, carbon, and amide proton NMR signals of Dpo4's catalytic core consisting of the finger, palm, and thumb domains. Our work provides the basis for further NMR spectroscopic studies of the interactions among Dpo4, DNA, and an incoming nucleotide. Keywords Sulfolobus solfataricus Dpo4 Á Catalytic core Á Y-family DNA polymerases Á Backbone resonance assignments Á Heteronuclear NMR The authors Dejian Ma and Jason D. Fowler, contributed equally.
Water plays essential structural and dynamical roles in protein-DNA recognition through contribut... more Water plays essential structural and dynamical roles in protein-DNA recognition through contributing to enthalpic or entropic stabilization of binding complex and by mediating intermolecular interactions and fluctuations for biological function. These interfacial water molecules are confined by the binding partners in nanospace but in many cases they are highly mobile and exchange with outside bulk solution. Here, we report our studies of the interfacial water dynamics in the binary and ternary complexes of a polymerase (Dpo4) with DNA and an incoming nucleotide using a site-specific tryptophan probe with femtosecond resolution. By systematic comparison of the interfacial water motions and local sidechain fluctuations in the apo, binary and ternary states of Dpo4, we observed that the DNA binding interface and active site is dynamically solvent accessible and the interfacial water dynamics are similar to the surface hydration water fluctuations on picosecond time scales. Our molecular dynamics simulations also show the binding interface full of water molecules and nonspecific weak interactions. Such a fluid binding interface facilitates the polymerase sliding on DNA for fast translocation while the spacious and mobile hydrated active site contributes to the low fidelity of the lesion-bypass Yfamily DNA polymerase.
Higher eukaryotes encode various Y-family DNA polymerases to perform global DNA lesion bypass. To... more Higher eukaryotes encode various Y-family DNA polymerases to perform global DNA lesion bypass. To provide complete mutation spectra for abasic lesion bypass, we employed short oligonucleotide sequencing assays to determine the sequences of abasic lesion bypass products synthesized by human Y-family DNA polymerases eta (hPolg), iota (hPoli) and kappa (hPoli). The fourth human Y-family DNA polymerase, Rev1, failed to generate full-length lesion bypass products after 3 h. The results indicate that hPoli generates mutations with a frequency from 10 to 80% during each nucleotide incorporation event. In contrast, hPolg is the least error prone, generating the fewest mutations in the vicinity of the abasic lesion and inserting dAMP with a frequency of 67% opposite the abasic site. While the error frequency of hPoli is intermediate to those of hPolg and hPoli, hPoli has the highest potential to create frameshift mutations opposite the abasic site. Moreover, the time (t 50 bypass) required to bypass 50% of the abasic lesions encountered by hPolg, hPoli and hPoli was 4.6, 112 and 1 823 s, respectively. These t 50 bypass values indicate that, among the enzymes, hPolg has the highest abasic lesion bypass efficiency. Together, our data suggest that hPolg is best suited to perform abasic lesion bypass in vivo.
Crystallographic studies of the C-terminal, DNA polymerase β-like domain of human DNA polymerase ... more Crystallographic studies of the C-terminal, DNA polymerase β-like domain of human DNA polymerase lambda (fPolλ) suggested that the catalytic cycle might not involve a large protein domain rearrangement as observed with several replicative DNA polymerases and DNA polymerase β. To examine solution-phase protein conformation changes in fPolλ, which also contains a breast cancer susceptibility gene 1 C-terminal domain and a Proline-rich domain at its N-terminus, we used a mass spectrometry-based protein footprinting approach. In parallel experiments, surface accessibility maps for Arg residues were compared for the free fPolλ versus the binary complex of enzyme•gapped DNA and the ternary complex of enzyme•gapped DNA•dNTP. These experiments suggested that fPolλ does not undergo major conformational changes during the catalysis in the solution phase. Furthermore, the mass spectrometry-based protein footprinting experiments revealed that active site residue R386 was shielded from the surface only in the presence of both a gapped DNA substrate and an incoming nucleotide dNTP. Site-directed mutagenesis and pre-steady state kinetic studies confirmed the importance of R386 for the enzyme activity, and indicated the key role for its guanidino group in stabilizing the negative charges of an incoming nucleotide and the leaving pyrophosphate product. We suggest that such interactions could be shared by and important for catalytic functions of other DNA polymerases.
Gemcitabine, 2-deoxy-2,2-difluorocytidine (dFdC), is a drug approved for use against various soli... more Gemcitabine, 2-deoxy-2,2-difluorocytidine (dFdC), is a drug approved for use against various solid tumors. Clinically, this moderately toxic nucleoside analog causes peripheral neuropathy, hematological dysfunction, and pulmonary toxicity in cancer patients. Although these side effects closely mimic symptoms of mitochondrial dysfunction, there is no direct evidence to show gemcitabine interferes with mitochondrial DNA replication catalyzed by human DNA polymerase ␥. Here we employed presteady state kinetic methods to directly investigate the incorporation of the 5-triphosphorylated form of gemcitabine (dFdCTP), the excision of the incorporated monophosphorylated form (dFdCMP), and the bypass of template base dFdC catalyzed by human DNA polymerase ␥. Opposite template base dG, dFdCTP was incorporated with a 432-fold lower efficiency than dCTP. Although dFdC is not a chain terminator, the incorporated dFdCMP decreased the incorporation efficiency of the next 2 correct nucleotides by 214-and 7-fold, respectively. Moreover, the primer 3-dFdCMP was excised with a 50-fold slower rate than the matched 3-dCMP. When dFdC was encountered as a template base, DNA polymerase ␥ paused at the lesion and one downstream position but eventually elongated the primer to full-length product. These pauses were because of a 1,000-fold decrease in nucleotide incorporation efficiency. Interestingly, the polymerase fidelity at these pause sites decreased by 2 orders of magnitude. Thus, our pre-steady state kinetic studies provide direct evidence demonstrating the inhibitory effect of gemcitabine on the activity of human mitochondrial DNA polymerase. Many nucleoside analogs are potent anti-cancer and antiviral drug compounds. Among 15 Food and Drug Administra
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Papers by Jason Fowler