Crystal structures of the BsPif1 helicase reveal that a major movement of the 2B SH3 domain is required for DNA unwinding

Pif1 helicases are ubiquitous members of the SF1B family and are essential for maintaining genome stability. It was speculated that Pif1-specific motifs may fold in specific structures, conferring distinct activities upon it. Here, we report the crystal structures of the Pif1 helicase from Bacteroides spp with and without adenosine triphosphate (ATP) analog/ssDNA. BsPif1 shares structural similarities with RecD2 and Dda helicases but has specific features in the 1B and 2B domains. The highly conserved Pif1 family specific sequence motif interacts with and constraints a putative pin-loop in domain 1B in a precise conformation. More importantly, we found that the 2B domain which contains a specific extended hairpin undergoes a significant rotation and/or movement upon ATP and DNA binding, which is absolutely required for DNA unwinding. We therefore propose a mechanism for DNA unwinding in which the 2B domain plays a predominant role. The fact that the conformational change regulates Pif1 activity may provide insight into the puzzling observation that Pif1 becomes highly processive during break-induced replication in association with Polδ, while the isolated Pif1 has low processivity.     

The T4 phage SF1B helicase Dda is structurally optimized to perform DNA strand separation

Helicases move on DNA via an ATP binding and hydrolysis mechanism coordinated by well-characterized helicase motifs. However, the translocation along single-stranded DNA (ssDNA) and the strand separation of double-stranded (dsDNA) may be loosely or tightly coupled. Dda is a phage T4 SF1B helicase with sequence homology to the Pif1 family of helicases that tightly couples translocation to strand separation. The crystal structure of the Dda-ssDNA binary complex reveals a domain referred to as the "pin" that was previously thought to remain static during strand separation. The pin contains a conserved phenylalanine that mediates a transient base-stacking interaction that is absolutely required for separation of dsDNA. The pin is secured at its tip by protein-protein interactions through an extended SH3 domain thereby creating a rigid strut. The conserved interface between the pin and the SH3 domain provides the mechanism for tight coupling of translocation to strand separation.     

The Bacteroides sp. 3_1_23 Pif1 protein is a multifunctional helicase

ScPif1 DNA helicase is the prototypical member of a 5′-to-3′ helicase superfamily conserved from bacteria to human and plays various roles in the maintenance of genomic homeostasis. While many studies have been performed with eukaryotic Pif1 helicases, including yeast and human Pif1 proteins, the potential functions and biochemical properties of prokaryotic Pif1 helicases remain largely unknown. Here, we report the expression, purification and biochemical analysis of Pif1 helicase from Bacteroides sp. 3_1_23 (BsPif1). BsPif1 binds to a large panel of DNA substrates and, in particular, efficiently unwinds partial duplex DNAs with 5′-overhang, fork-like substrates, D-loop and flap-like substrates, suggesting that BsPif1 may act at stalled DNA replication forks and enhance Okazaki fragment maturation. Like its eukaryotic homologues, BsPif1 resolves R-loop structures and unwinds DNA–RNA hybrids. Furthermore, BsPif1 efficiently unfolds G-quadruplexes and disrupts nucleoprotein complexes. Altogether, these results highlight that prokaryotic Pif1 helicases may resolve common issues that arise during DNA transactions. Interestingly, we found that BsPif1 is different from yeast Pif1, but resembles more human Pif1 with regard to substrate specificity, helicase activity and mode of action. These findings are discussed in the context of the possible functions of prokaryotic Pif1 helicases in vivo.     

G-quadruplexes Significantly Stimulate Pif1 Helicase-catalyzed Duplex DNA Unwinding

The evolutionarily conserved G-quadruplexes (G4s) are faithfully inherited and serve a variety of cellular functions such as telomere maintenance, gene regulation, DNA replication initiation, and epigenetic regulation. Different from the Watson-Crick base-pairing found in duplex DNA, G4s are formed via Hoogsteen base pairing and are very stable and compact DNA structures. Failure of untangling them in the cell impedes DNA-based transactions and leads to genome instability. Cells have evolved highly specific helicases to resolve G4 structures. We used a recombinant nuclear form of Saccharomyces cerevisiae Pif1 to characterize Pif1-mediated DNA unwinding with a substrate mimicking an ongoing lagging strand synthesis stalled by G4s, which resembles a replication origin and a G4-structured flap in Okazaki fragment maturation. We find that the presence of G4 may greatly stimulate the Pif1 helicase to unwind duplex DNA. Further studies reveal that this stimulation results from G4-enhanced Pif1 dimerization, which is required for duplex DNA unwinding. This finding provides new insights into the properties and functions of G4s. We discuss the observed activation phenomenon in relation to the possible regulatory role of G4s in the rapid rescue of the stalled lagging strand synthesis by helping the replicator recognize and activate the replication origin as well as by quickly removing the G4-structured flap during Okazaki fragment maturation.     

The Pif1p subfamily of helicases: region-specific DNA helicases?

DNA helicases are required for DNA replication, recombination and repair. Despite a common enzymatic function - the ability to unwind duplex DNA - most helicases share only limited amino acid sequence similarity. Helicases that have significant sequence similarity define a subfamily. It remains unclear, however, how this sequence similarity relates to helicase function. The Saccharomyces cerevisiae Pif1p helicase is the prototype member of a helicase subfamily that is conserved from yeasts to humans. As the two Pif1p subfamily members studied to date affect the same DNA sequences, the amino acid similarity that defines this subfamily might reflect common substrates.     

Regulation of translocation polarity by helicase domain 1 in SF2B helicases

Structurally similar superfamily I (SF1) and II (SF2) helicases translocate on single-stranded DNA (ssDNA) with defined polarity either in the 5'-3' or in the 3'-5' direction. Both 5'-3' and 3'-5' translocating helicases contain the same motor core comprising two RecA-like folds. SF1 helicases of opposite polarity bind ssDNA with the same orientation, and translocate in opposite directions by employing a reverse sequence of the conformational changes within the motor domains. Here, using proteolytic DNA and mutational analysis, we have determined that SF2B helicases bind ssDNA with the same orientation as their 3'-5' counterparts. Further, 5'-3' translocation polarity requires conserved residues in HD1 and the FeS cluster containing domain. Finally, we propose the FeS cluster-containing domain also provides a wedge-like feature that is the point of duplex separation during unwinding.     

E. coli Rep oligomers are required to initiate DNA unwinding in vitro.

E. coli Rep protein is a 3' to 5' SF1 superfamily DNA helicase which is monomeric in the absence of DNA, but can dimerize upon binding either single-stranded or duplex DNA. A variety of biochemical studies have led to proposals that Rep dimerization is important for its helicase activity; however, recent structural studies of Bacillus stearothermophilus PcrA have led to suggestions that SF1 helicases, such as E. coli Rep and E. coli UvrD, function as monomeric helicases. We have examined the question of whether Rep oligomerization is important for its DNA helicase activity using pre-steady state stopped-flow and chemical quenched-flow kinetic studies of Rep-catalyzed DNA unwinding. The results from four independent experiments demonstrate that Rep oligomerization is required for initiation of DNA helicase activity in vitro. No DNA unwinding is observed when only a Rep monomer is bound to the DNA substrate, even when fluorescent DNA substrates are used that can detect partial unwinding of the first few base-pairs at the ss-ds-DNA junction. In fact, under these conditions, ATP hydrolysis causes dissociation of the Rep monomer from the DNA, rather than DNA unwinding. These studies demonstrate that wild-type Rep monomers are unable to initiate DNA unwinding in vitro, and that oligomerization is required.     

RPA alleviates the inhibitory effect of vinylphosphonate internucleotide linkages on DNA unwinding by BLM and WRN helicases

Bloom (BLM) and Werner (WRN) syndrome proteins are members of the RecQ family of SF2 DNA helicases. In this paper, we show that restricting the rotational DNA backbone flexibility, by introducing vinylphosphonate internucleotide linkages in the translocating DNA strand, inhibits efficient duplex unwinding by these enzymes. The human single-stranded DNA binding protein replication protein A (RPA) fully restores the unwinding activity of BLM and WRN on vinylphosphonate-containing substrates while the heterologous single-stranded DNA binding protein from Escherichia coli (SSB) restores the activity only partially. Both RPA and SSB fail to restore the unwinding activity of the SF1 PcrA helicase on modified substrates, implying specific interactions of RPA with the BLM and WRN helicases. Our data highlight subtle differences between SF1 and SF2 helicases and suggest that although RecQ helicases belong to the SF2 family, they are mechanistically more similar to the SF1 PcrA helicase than to other SF2 helicases that are not affected by vinylphosphonate modifications.     

DNA-binding orientation and domain conformation of the E. coli rep helicase monomer bound to a partial duplex junction: single-molecule studies of fluorescently labeled enzymes

The SF1 DNA helicases are multi-domain proteins that can unwind duplex DNA in reactions that are coupled to ATP binding and hydrolysis. Crystal structures of two such helicases, Escherichia coli Rep and Bacillus stearothermophilus PcrA, show that the 2B sub-domain of these proteins can be found in dramatically different orientations (closed versus open) with respect to the remainder of the protein, suggesting that the 2B domain is highly flexible. By systematically using fluorescence resonance energy transfer at the single-molecule level, we have determined both the orientation of an E.coli Rep monomer bound to a 3'-single-stranded-double-stranded (ss/ds) DNA junction in solution, as well as the relative orientation of its 2B sub-domain. To accomplish this, we developed a highly efficient procedure for site-specific fluorescence labeling of Rep and a bio-friendly immobilization scheme, which preserves its activities. Both ensemble and single-molecule experiments were carried out, although the single-molecule experiments proved to be essential here in providing quantitative distance information that could not be obtained by steady-state ensemble measurements. Using distance-constrained triangulation procedures we demonstrate that in solution the 2B sub-domain of a Rep monomer is primarily in the "closed" conformation when bound to a 3'-ss/ds DNA, similar to the orientation observed in the complex of PcrA bound to a 3'-ss/ds DNA. Previous biochemical studies have shown that a Rep monomer bound to such a 3'-ss/ds DNA substrate is unable to unwind the DNA and that a Rep oligomer is required for helicase activity. Therefore, the closed form of Rep bound to a partial duplex DNA appears to be an inhibited form of the enzyme.     

Yeast Helicase Pif1 Unwinds RNA:DNA Hybrids with Higher Processivity than DNA:DNA Duplexes

Saccharomyces cerevisiae Pif1, an SF1B helicase, has been implicated in both mitochondrial and nuclear functions. Here we have characterized the preference of Pif1 for RNA:DNA heteroduplexes in vitro by investigating several kinetic parameters associated with unwinding. We show that the preferential unwinding of RNA:DNA hybrids is due to neither specific binding nor differences in the rate of strand separation. Instead, Pif1 is capable of unwinding RNA:DNA heteroduplexes with moderately greater processivity compared with its duplex DNA:DNA counterparts. This higher processivity of Pif1 is attributed to slower dissociation from RNA:DNA hybrids. Biologically, this preferential role of the helicase may contribute to its functions at both telomeric and nontelomeric sites.     

Yeast Pif1 Helicase Exhibits a One-base-pair Stepping Mechanism for Unwinding Duplex DNA

Kinetic analysis of the DNA unwinding and translocation activities of helicases is necessary for characterization of the biochemical mechanism(s) for this class of enzymes. Saccharomyces cerevisiae Pif1 helicase was characterized using presteady state kinetics to determine rates of DNA unwinding, displacement of streptavidin from biotinylated DNA, translocation on single-stranded DNA (ssDNA), and ATP hydrolysis activities. Unwinding of substrates containing varying duplex lengths was fit globally to a model for stepwise unwinding and resulted in an unwinding rate of ∼75 bp/s and a kinetic step size of 1 base pair. Pif1 is capable of displacing streptavidin from biotinylated oligonucleotides with a linear increase in the rates as the length of the oligonucleotides increased. The rate of translocation on ssDNA was determined by measuring dissociation from varying lengths of ssDNA and is essentially the same as the rate of unwinding of dsDNA, making Pif1 an active helicase. The ATPase activity of Pif1 on ssDNA was determined using fluorescently labeled phosphate-binding protein to measure the rate of phosphate release. The quantity of phosphate released corresponds to a chemical efficiency of 0.84 ATP/nucleotides translocated. Hence, when all of the kinetic data are considered, Pif1 appears to move along DNA in single nucleotide or base pair steps, powered by hydrolysis of 1 molecule of ATP.     

To peep into Pif1 helicase: Multifaceted all the way from genome stability to repair-associated DNA synthesis

Pif1 DNA helicase is the prototypical member of a 5′ to 3′ helicase superfamily conserved from bacteria to humans. In Saccharomyces cerevisiae, Pif1 and its homologue Rrm3, localize in both mitochondria and nucleus playing multiple roles in the maintenance of genomic homeostasis. They display relatively weak processivities in vitro, but have largely non-overlapping functions on common genomic loci such as mitochondrial DNA, telomeric ends, and many replication forks especially at hard-to-replicate regions including ribosomal DNA and G-quadruplex structures. Recently, emerging evidence shows that Pif1, but not Rrm3, has a significant new role in repair-associated DNA synthesis with Polδ during homologous recombination stimulating D-loop migration for conservative DNA replication. Comparative genetic and biochemical studies on the structure and function of Pif1 family helicases across different biological systems are further needed to elucidate both diversity and specificity of their mechanisms of action that contribute to genome stability.     

Novel Function of the Fanconi Anemia Group J or RECQ1 Helicase to Disrupt Protein-DNA Complexes in a Replication Protein A-stimulated Manner

Understanding how cellular machinery deals with chromosomal genome complexity is an important question because protein bound to DNA may affect various cellular processes of nucleic acid metabolism. DNA helicases are at the forefront of such processes, yet there is only limited knowledge how they remodel protein-DNA complexes and how these mechanisms are regulated. We have determined that representative human RecQ and Fe-S cluster DNA helicases are potently blocked by a protein-DNA interaction. The Fanconi anemia group J (FANCJ) helicase partners with the single-stranded DNA-binding protein replication protein A (RPA) to displace BamHI-E111A bound to duplex DNA in a specific manner. Protein displacement was dependent on the ATPase-driven function of the helicase and unique properties of RPA. Further biochemical studies demonstrated that the shelterin proteins TRF1 and TRF2, which preferentially bind the telomeric repeat found at chromosome ends, effectively block FANCJ from unwinding the forked duplex telomeric substrate. RPA, but not the Escherichia coli single-stranded DNA-binding protein or shelterin factor Pot1, stimulated FANCJ ejection of TRF1 from the telomeric DNA substrate. FANCJ was also able to displace TRF2 from the telomeric substrate in an RPA-dependent manner. The stimulation of helicase-catalyzed protein displacement is also observed with the DNA helicase RECQ1, suggesting a conserved functional interaction of RPA-interacting helicases. These findings suggest that partnerships between RPA and interacting human DNA helicases may greatly enhance their ability to dislodge proteins bound to duplex DNA, an activity that is likely to be highly relevant to their biological roles in DNA metabolism.     

The absence of Top3 reveals an interaction between the Sgs1 and Pif1 DNA helicases in Saccharomyces cerevisiae

RecQ DNA helicases and Topo III topoisomerases have conserved genetic, physical, and functional interactions that are consistent with a model in which RecQ creates a recombination-dependent substrate that is resolved by Topo III. The phenotype associated with Topo III loss suggests that accumulation of a RecQ-created substrate is detrimental. In yeast, mutation of the TOP3 gene encoding Topo III causes pleiotropic defects that are suppressed by deletion of the RecQ homolog Sgs1. We searched for gene dosage suppressors of top3 and identified Pif1, a DNA helicase that acts with polarity opposite to that of Sgs1. Pif1 overexpression suppresses multiple top3 defects, but exacerbates sgs1 and sgs1 top3 defects. Furthermore, Pif1 helicase activity is essential in the absence of Top3 in an Sgs1-dependent manner. These data clearly demonstrate that Pif1 helicase activity is required to counteract Sgs1 helicase activity that has become uncoupled from Top3. Pif1 genetic interactions with the Sgs1-Top3 pathway are dependent upon homologous recombination. We also find that Pif1 is recruited to DNA repair foci and that the frequency of these foci is significantly increased in top3 mutants. Our results support a model in which Pif1 has a direct role in the prevention or repair of Sgs1-induced DNA damage that accumulates in top3 mutants.     

Crystallographic structure of the amino terminal domain of yeast initiation factor 4A, a representative DEAD-box RNA helicase

The eukaryotic translation initiation factor 4A (elF4A) is a representative of the DEAD-box RNA helicase protein family. We have solved the crystallographic structure of the amino-terminal domain (residues 1-223) of yeast elF4A. The domain is built around a core scaffold, a parallel alpha-beta motif with five beta strands, that is found in other RNA and DNA helicases, as well as in the RecA protein. The amino acid sequence motifs that are conserved within the helicase family are localized to the beta strand-->alpha helix junctions within the core. The core of the amino terminal domain of elF4A is amplified with additional structural elements that differ from those of other helicases. The phosphate binding loop (the Walker A motif) is in an unusual closed conformation. The crystallographic structure reveals specific interactions between amino acid residues of the phosphate binding loop, the DEAD motif, and the SAT motif, whose alteration is known to impair coupling between the ATPase cycle and the RNA unwinding activity of elF4A.     

Residues within the B' motif are critical for DNA binding by the superfamily 3 helicase Rep40 of adeno-associated virus type 2

We have recently published the crystal structure of the adeno-associated virus type 2 superfamily 3 (SF3) helicase Rep40. Although based on its biochemical properties it is unlikely that Rep40 plays a central role as a replicative helicase the involvement of this motor protein in DNA packaging has recently been demonstrated. Here we focused our attention on residues that fall within and adjacent to the B' motif of SF3 helicases that directly interact with single-stranded DNA during translocation of the motor protein. In vitro, alanine substitution at positions Lys-404 or Lys-406 abrogated the ability of the protein to interact with single-stranded DNA as demonstrated by electrophoretic mobility shift assay and fluorescence anisotropy, and accordingly these mutants could not unwind a partially duplex DNA substrate. Despite this loss of helicase activity, basal ATPase activity in these mutants remained intact. However, unlike the wild-type protein, K404A and K406A ATPase activity was not stimulated by DNA. As predicted, disruption of motor activity through interference with DNA binding resulted in an inability of Rep40 to package adeno-associated virus DNA in a tissue culture-based assay. Taken together, we characterized, for the first time in an SF3 helicase family member, residues that are directly involved in single-stranded DNA binding and that are critical for the Rep motor activity. Based on our findings we propose B' as the signature motif of SF3 helicases that is responsible for the complex interactions required for the coupling of DNA binding and ATP hydrolysis.     

G-quadruplex and G-rich sequence stimulate Pif1p-catalyzed downstream duplex DNA unwinding through reducing waiting time at ss/dsDNA junction

Alternative DNA structures that deviate from B-form double-stranded DNA such as G-quadruplex (G4) DNA can be formed by G-rich sequences that are widely distributed throughout the human genome. We have previously shown that Pif1p not only unfolds G4, but also unwinds the downstream duplex DNA in a G4-stimulated manner. In the present study, we further characterized the G4-stimulated duplex DNA unwinding phenomenon by means of single-molecule fluorescence resonance energy transfer. It was found that Pif1p did not unwind the partial duplex DNA immediately after unfolding the upstream G4 structure, but rather, it would dwell at the ss/dsDNA junction with a ‘waiting time’. Further studies revealed that the waiting time was in fact related to a protein dimerization process that was sensitive to ssDNA sequence and would become rapid if the sequence is G-rich. Furthermore, we identified that the G-rich sequence, as the G4 structure, equally stimulates duplex DNA unwinding. The present work sheds new light on the molecular mechanism by which G4-unwinding helicase Pif1p resolves physiological G4/duplex DNA structures in cells.     

嗜热脱铁去硫弧菌Pif1解旋酶生物学活性的分子特征分析

已知Pif1解旋酶在维持基因组稳定性方面发挥重要作用,且不同生物Pif1解旋酶具有不同的生物学活性;然而迄今为止,嗜热细菌Pif1解旋酶的生物学活性分子特征的研究尚未见报道。本文运用生物化学与生物物理学前沿技术,系统地研究了嗜热脱铁去硫弧菌Pif1解旋酶（Defe.Pif1）结合活性与解旋活性的分子特征。通过原核表达纯化系统,本研究获得纯度95%以上、无标签的Defe.Pif1蛋白。利用荧光偏振技术研究Defe.Pif1结合反应的底物特异性,揭示出Defe.Pif1优先结合ssDNA与G4 DNA,并对含3′-尾链的部分双链底物有较强亲和力,其结合反应底物特异性为：ssDNA > G4 DNA > 3′-ssDNA-dsDNA≈Y-structure > Other substrates。通过快速停留检测技术研究Defe.Pif1的解旋活性,明确其最适解旋温度为50℃,最佳反应溶液为10 mmol/L NaCl、3 mmol/L DTT、3 mmol/L MgCl2及1 mmol/L ATP;进一步的解旋动力学特征分析结果显示,Defe.Pif1可以高效解旋含G4结构的DNA底物,其解旋5′-G4 dsDNA底物时的解旋幅度超过90%,解旋速率也与其解旋5′-ss-dsDNA底物的速率相近,提示Defe.Pif1解旋G4 DNA更接近Bs.Pif1的单体反应模式。此外,本研究还发现Defe.Pif1解旋不同类型复制叉/复制泡底物时拥有独特的解旋倾向性：与解旋其它复制中间体DNA的低效性不同,Defe.Pif1解旋12nt bubble底物的速率与幅度均较高,暗示12nt bubble结构很可能是该解旋酶复制中间体的天然底物。上述结果证明,Defe.Pif1不仅具有Pif1解旋酶家族成员共同的结合与解旋G4 DNA等活性特征;而且作为嗜热细菌的解旋酶,它还具有独特的反应条件与解旋底物特异性。本研究为研究Pif1解旋酶家族其它成员的分子特征与生物功能提供了潜在的研究策略,为阐明此类Pif1解旋酶的分子作用机制奠定实验基础。     

Coupling DNA-binding and ATP hydrolysis in Escherichia coli RecQ: role of a highly conserved aromatic-rich sequence

RecQ enzymes are broadly conserved Superfamily-2 (SF-2) DNA helicases that play critical roles in DNA metabolism. RecQ proteins use the energy of ATP hydrolysis to drive DNA unwinding; however, the mechanisms by which RecQ links ATPase activity to DNA-binding/unwinding are unknown. In many Superfamily-1 (SF-1) DNA helicases, helicase sequence motif III links these activities by binding both single-stranded (ss) DNA and ATP. However, the ssDNA-binding aromatic-rich element in motif III present in these enzymes is missing from SF-2 helicases, raising the question of how these enzymes link ATP hydrolysis to DNA-binding/unwinding. We show that Escherichia coli RecQ contains a conserved aromatic-rich loop in its helicase domain between motifs II and III. Although placement of the RecQ aromatic-rich loop is topologically distinct relative to the SF-1 enzymes, both loops map to similar tertiary structural positions. We examined the functions of the E.coli RecQ aromatic-rich loop using RecQ variants with single amino acid substitutions within the segment. Our results indicate that the aromatic-rich loop in RecQ is critical for coupling ATPase and DNA-binding/unwinding activities. Our studies also suggest that RecQ's aromatic-rich loop might couple ATP hydrolysis to DNA-binding in a mechanistically distinct manner from SF-1 helicases.     

Escherichia coli DNA helicases: mechanisms of DNA unwinding

DNA helicases are ubiquitous enzymes that catalyse the unwinding of duplex DNA during replication, recombination and repair. These enzymes have been studied extensively; however, the specific details of how any helicase unwinds duplex DNA are unknown. Although it is clear that not all helicases unwind duplex DNA in an identical way, many helicases possess similar properties, which are thus likely to be of general importance to their mechanism of action. For example, since helicases appear generally to be oligomeric enzymes, the hypothesis is presented in this review that the functionally active forms of DNA helicases are oligomeric. The oligomeric nature of helicases provides them with multiple DNA-binding sites, allowing the transient formation of ternary structures, such that at an unwinding fork, the helicase can bind either single-stranded and duplex DNA simultaneously or two strands of single-stranded DNA. Modulation of the relative affinities of these binding sites for single-stranded versus duplex DNA through ATP binding and hydrolysis would then provide the basis for a cycling mechanism for processive unwinding of DNA by helicases. The properties of the Escherichia coli DNA helicases are reviewed and possible mechanisms by which helicases might unwind duplex DNA are discussed in view of their oligomeric structures, with emphasis on the E. coli Rep, RecBCD and phage T7 gene 4 helicases.