Single-Molecule Investigation of Response to Oxidative DNA Damage by a Y-Family DNA Polymerase

Processing oxidatively damaged bases at DNA strand breaks by APE1

Reactive oxygen species attack the structure of DNA, thus altering its base-pairing properties. Consequently, oxidative stress-associated DNA lesions are a major source of the mutation load that gives rise to cancer and other diseases. Base excision repair (BER) is the pathway primarily tasked with repairing DNA base damage, with apurinic/apyrimidinic endonuclease (APE1) having both AP-endonuclease and 3' to 5' exonuclease (exo) DNA cleavage functions. The lesion 8-oxo-7,8-dihydroguanine (8-oxoG) can enter the genome as either a product of direct damage to the DNA, or through polymerase insertion at the 3'-end of a DNA strand during replication or repair. Importantly, 3'-8-oxoG impairs the ligation step of BER and therefore must be removed by the exo activity of a surrogate enzyme to prevent double stranded breaks and cell death. In the present study, we use X-ray crystallography to characterize the exo activity of APE1 on 3'-8-oxoG substrates. These structures support a unified APE1 exo mechanism that differs from its more canonical AP-endonuclease activity. In addition, through complementation of the structural data with enzyme kinetics and binding studies employing both wild-type and rationally designed APE1 mutants, we were able to identify and characterize unique protein: DNA contacts that specifically mediate 8-oxoG removal by APE1.

Beyond the Lesion: Back to High Fidelity DNA Synthesis

High fidelity (HiFi) DNA polymerases (Pols) perform the bulk of DNA synthesis required to duplicate genomes in all forms of life. Their structural features, enzymatic mechanisms, and inherent properties are well-described over several decades of research. HiFi Pols are so accurate that they become stalled at sites of DNA damage or lesions that are not one of the four canonical DNA bases. Once stalled, the replisome becomes compromised and vulnerable to further DNA damage. One mechanism to relieve stalling is to recruit a translesion synthesis (TLS) Pol to rapidly synthesize over and past the damage. These TLS Pols have good specificities for the lesion but are less accurate when synthesizing opposite undamaged DNA, and so, mechanisms are needed to limit TLS Pol synthesis and recruit back a HiFi Pol to reestablish the replisome. The overall TLS process can be complicated with several cellular Pols, multifaceted protein contacts, and variable nucleotide incorporation kinetics all contributing to several discrete substitution (or template hand-off) steps. In this review, we highlight the mechanistic differences between distributive equilibrium exchange events and concerted contact-dependent switching by DNA Pols for insertion, extension, and resumption of high-fidelity synthesis beyond the lesion.

Investigating the trade-off between folding and function in a multidomain Y-family DNA polymerase

The way in which multidomain proteins fold has been a puzzling question for decades. Until now, the mechanisms and functions of domain interactions involved in multidomain protein folding have been obscure. Here, we develop structure-based models to investigate the folding and DNA-binding processes of the multidomain Y-family DNA polymerase IV (DPO4). We uncover shifts in the folding mechanism among ordered domain-wise folding, backtracking folding, and cooperative folding, modulated by interdomain interactions. These lead to ‘U-shaped’ DPO4 folding kinetics. We characterize the effects of interdomain flexibility on the promotion of DPO4–DNA (un)binding, which probably contributes to the ability of DPO4 to bypass DNA lesions, which is a known biological role of Y-family polymerases. We suggest that the native topology of DPO4 leads to a trade-off between fast, stable folding and tight functional DNA binding. Our approach provides an effective way to quantitatively correlate the roles of protein interactions in conformational dynamics at the multidomain level.

Differential mechanisms of tolerance to extreme environmental conditions in tardigrades

Tardigrades, also known as water bears, are small aquatic animals that inhabit marine, fresh water or limno-terrestrial environments. While all tardigrades require surrounding water to grow and reproduce, species living in limno-terrestrial environments (e.g. Ramazzottius varieornatus) are able to undergo almost complete dehydration by entering an arrested state known as anhydrobiosis, which allows them to tolerate ionic radiation, extreme temperatures and intense pressure. Previous studies based on comparison of the genomes of R. varieornatus and Hypsibius dujardini - a less tolerant tardigrade - have pointed to potential mechanisms that may partially contribute to their remarkable ability to resist extreme physical conditions. In this work, we have further annotated the genomes of both tardigrades using a guided approach in search for novel mechanisms underlying the extremotolerance of R. varieornatus. We have found specific amplifications of several genes, including MRE11 and XPC, and numerous missense variants exclusive of R. varieornatus in CHEK1, POLK, UNG and TERT, all of them involved in important pathways for DNA repair and telomere maintenance. Taken collectively, these results point to genomic features that may contribute to the enhanced ability to resist extreme environmental conditions shown by R. varieornatus.

Quantifying the Intrinsic Conformation Energy Landscape Topography of Proteins with Large-Scale Open-Closed Transition

Large-scale conformational changes of proteins, including the open-closed transitions, are crucial for a variety of protein functions. These open-closed transitions are often associated with ligand binding. However, the understandings of the underlying mechanisms of the conformational changes within proteins during the open-closed transitions are still challenging at present. In this study, we quantified the intrinsic underlying conformational energy landscapes of five different proteins with large-scale open-closed transitions. This is realized by exploring the underlying density of states and the intrinsic conformational energy landscape topography measure Λ. Λ is a dimensionless ratio of conformational energy gap δE versus conformational energy roughness δE and configurational entropy S or size of the intrinsic conformational energy landscape. By quantifying the Λ of intrinsic open-closed conformational (Λ^oc) and intrinsic global folding (Λ^global) energy landscapes, we show that both intrinsic open-closed conformation energy and entropy landscapes are funneled toward the closed state. Furthermore, our results indicate the strong correlations between Λ and thermodynamics (conformational state transition temperature against trapping temperature) as well as between Λ and kinetics (open-closed kinetic time) of these proteins. This shows that the intrinsic conformational landscape topography determines both the conformational thermodynamic stability and kinetic speed of the conformational dynamics. Our investigations provide important insights for understanding the fundamental mechanisms of the protein conformational dynamics in a physical and global way.

Investigation of sliding DNA clamp dynamics by single-molecule fluorescence, mass spectrometry and structure-based modeling

Proliferating cell nuclear antigen (PCNA) is a trimeric ring-shaped clamp protein that encircles DNA and interacts with many proteins involved in DNA replication and repair. Despite extensive structural work to characterize the monomeric, dimeric, and trimeric forms of PCNA alone and in complex with interacting proteins, no structure of PCNA in a ring-open conformation has been published. Here, we use a multidisciplinary approach, including single-molecule Förster resonance energy transfer (smFRET), native ion mobility-mass spectrometry (IM-MS), and structure-based computational modeling, to explore the conformational dynamics of a model PCNA from Sulfolobus solfataricus (Sso), an archaeon. We found that Sso PCNA samples ring-open and ring-closed conformations even in the absence of its clamp loader complex, replication factor C, and transition to the ring-open conformation is modulated by the ionic strength of the solution. The IM-MS results corroborate the smFRET findings suggesting that PCNA dynamics are maintained in the gas phase and further establishing IM-MS as a reliable strategy to investigate macromolecular motions. Our molecular dynamic simulations agree with the experimental data and reveal that ring-open PCNA often adopts an out-of-plane left-hand geometry. Collectively, these results implore future studies to define the roles of PCNA dynamics in DNA loading and other PCNA-mediated interactions.

Functional Insights Revealed by the Kinetic Mechanism of CRISPR/Cas9

The discovery of prokaryotic adaptive immunity prompted widespread use of the RNA-guided clustered regularly interspaced short palindromic repeat (CRISPR)-associated (Cas) endonuclease Cas9 for genetic engineering. However, its kinetic mechanism remains undefined, and details of DNA cleavage are poorly characterized. Here, we establish a kinetic mechanism of Streptococcus pyogenes Cas9 from guide-RNA binding through DNA cleavage and product release. Association of DNA to the binary complex of Cas9 and guide-RNA is rate-limiting during the first catalytic turnover, while DNA cleavage from a pre-formed ternary complex of Cas9, guide-RNA, and DNA is rapid. Moreover, an extremely slow release of DNA products essentially restricts Cas9 to be a single-turnover enzyme. By simultaneously measuring the contributions of the HNH and RuvC nuclease activities of Cas9 to DNA cleavage, we also uncovered the kinetic basis by which HNH conformationally regulates the RuvC cleavage activity. Together, our results provide crucial kinetic and functional details regarding Cas9 which will inform gene-editing experiments, guide future research to understand off-target DNA cleavage by Cas9, and aid in the continued development of Cas9 as a biotechnological tool.

Influence of DNA Lesions on Polymerase-Mediated DNA Replication at Single-Molecule Resolution

Faithful replication of DNA is a critical aspect in maintaining genome integrity. DNA polymerases are responsible for replicating DNA, and high-fidelity polymerases do this rapidly and at low error rates. Upon exposure to exogenous or endogenous substances, DNA can become damaged and this can alter the speed and fidelity of a DNA polymerase. In this instance, DNA polymerases are confronted with an obstacle that can result in genomic instability during replication, for example, by nucleotide misinsertion or replication fork collapse. It is important to know how DNA polymerases respond to damaged DNA substrates to understand the mechanism of mutagenesis and chemical carcinogenesis. Single-molecule techniques have helped to improve our current understanding of DNA polymerase-mediated DNA replication, as they enable the dissection of mechanistic details that can otherwise be lost in ensemble-averaged experiments. These techniques have also been used to gain a deeper understanding of how single DNA polymerases behave at the site of the damage in a DNA substrate. In this review, we evaluate single-molecule studies that have examined the interaction between DNA polymerases and damaged sites on a DNA template.

Increased Processivity, Misincorporation, and Nucleotide Incorporation Efficiency in Sulfolobus solfataricus Dpo4 Thumb Domain Mutants

The present study aimed to increase the processivity of Sulfolobus solfataricus DNA polymerase Dpo4. Protein engineering and bioinformatics were used to compile a library of potential Dpo4 mutation sites. Ten potential mutants were identified and constructed. A primer extension assay was used to evaluate the processivity of Dpo4 mutants. Thumb (A181D) and finger (E63K) domain mutants showed a processivity of 20 and 19 nucleotides (nt), respectively. A little finger domain mutant (I248Y) exhibited a processivity of 17 nt, only 1 nt more than wild-type Dpo4. Furthermore, the A181D mutant showed lower fidelity and higher nucleotide incorporation efficiency (4.74 × 10^-4 s^-1 μM^-1) than E63K and I248Y mutants. When tasked with bypassing damage, the A181D mutant exhibited a 3.81-fold and 2.62-fold higher catalytic efficiency (k_cat/K_m ) at incorporating dCTP and dATP, respectively, than wild-type Dpo4. It also showed a 55% and 91.5% higher catalytic efficiency when moving beyond the damaged 8-oxoG:C and 8-oxoG:A base pairs, respectively, compared to wild-type Dpo4. Protein engineering and bioinformatics methods can effectively increase the processivity and translesion synthesis ability of Dpo4.IMPORTANCE DNA polymerases with poor fidelity can be exploited to store data and record changes in response to the intracellular environment. Sulfolobus solfataricus Dpo4 is such an enzyme, although its use is hindered by its low processivity. In this work, we used a bioinformatics and protein engineering approach to generate Dpo4 mutants with improved processivity. We identified the Dpo4 thumb domain as the most relevant in controlling processivity.

Backbone assignment of the binary complex of the full length Sulfolobus solfataricus DNA polymerase IV and DNA

Sulfolobus solfataricus DNA polymerase IV (Dpo4), a model Y-family DNA polymerase, bypasses a wide range of DNA lesions in vitro and in vivo. In this paper, we report the backbone chemical shift assignments of the full length Dpo4 in its binary complex with a 14/14-mer DNA substrate. Upon DNA binding, several β-stranded regions in the isolated catalytic core and little finger/linker fragments of Dpo4 become more structured. This work serves as a foundation for our ongoing investigation of conformational dynamics of Dpo4 and future determination of the first solution structures of a DNA polymerase and its binary and ternary complexes.

Advances in Structural and Single-Molecule Methods for Investigating DNA Lesion Bypass and Repair Polymerases

Innovative advances in X-ray crystallography and single-molecule biophysics have yielded unprecedented insight into the mechanisms of DNA lesion bypass and damage repair. Time-dependent X-ray crystallography has been successfully applied to view the bypass of 8-oxo-7,8-dihydro-2'-deoxyguanine (8-oxoG), a major oxidative DNA lesion, and the incorporation of the triphosphate form, 8-oxo-dGTP, catalyzed by human DNA polymerase β. Significant findings of these studies are highlighted here, and their contributions to the current mechanistic understanding of mutagenic translesion DNA synthesis (TLS) and base excision repair are discussed. In addition, single-molecule Förster resonance energy transfer (smFRET) techniques have recently been adapted to investigate nucleotide binding and incorporation opposite undamaged dG and 8-oxoG by Sulfolobus solfataricus DNA polymerase IV (Dpo4), a model Y-family DNA polymerase. The mechanistic response of Dpo4 to a DNA lesion and the complex smFRET technique are described here. In this perspective, we also describe how time-dependent X-ray crystallography and smFRET can be used to achieve the spatial and temporal resolutions necessary to answer some of the mechanistic questions that remain in the fields of TLS and DNA damage repair.

Investigation of Intradomain Motions of a Y-Family DNA Polymerase during Substrate Binding and Catalysis

DNA polymerases catalyze DNA synthesis through a stepwise kinetic mechanism that begins with binding to DNA, followed by selection, binding, and incorporation of a nucleotide into an elongating primer. It is hypothesized that subtle active site adjustments in a polymerase to align reactive moieties limit the rate of correct nucleotide incorporation. DNA damage can impede this process for many DNA polymerases, causing replication fork stalling, genetic mutations, and potentially cell death. However, specialized Y-family DNA polymerases are structurally evolved to efficiently bypass DNA damage in vivo, albeit at the expense of replication fidelity. Dpo4, a model Y-family polymerase from Sulfolobus solfataricus, has been well-studied kinetically, structurally, and computationally, which yielded a mechanistic understanding of how the Y-family DNA polymerases achieve their unique catalytic properties. We previously employed a real-time Förster resonance energy transfer (FRET) technique to characterize the global conformational motions of Dpo4 during DNA binding as well as nucleotide binding and incorporation by monitoring changes in distance between sites on the polymerase and DNA, and even between domains of Dpo4. Here, we extend the utility of our FRET methodology to observe conformational transitions within individual domains of Dpo4 during DNA binding and nucleotide incorporation. The results of this novel, intradomain FRET approach unify findings from many studies to fully clarify the complex DNA binding mechanism of Dpo4. Furthermore, intradomain motions in the Finger domain during nucleotide binding and incorporation, for the first time, report on the rate-limiting step of a single-nucleotide addition catalyzed by Dpo4.