Domain analysis of an archaeal RadA protein for the strand exchange activity

Comparative analysis of structural dynamics and allosteric mechanisms of RecA/Rad51 family proteins: Integrated atomistic MD simulation and network-based analysis

Homologous recombination plays a key role in double-strand break repair, stalled replication fork repair, and meiosis. The RecA/Rad51 family recombinases catalyze the DNA strand invasion reaction that occurs during homologous recombination. However, the high sequence differences between homologous groups have hindered the thoroughly studies of this ancient protein family. The dynamic mechanisms of the family, particularly at the residual level, remain poorly understood. In this work, five representative RecA/Rad51 recombinase family members from all major kingdoms of living organisms: prokaryotes, eukaryotes, archaea, and viruses, were selected to explore the molecular mechanisms behind their conserved biological significance. A variety of techniques, including all-atom molecular dynamics simulation, perturbation response scanning, and protein structure network analysis, were used to examine the flexibility and correlation of protein domains, distribution of sensors and effectors and conserved hub residues. Furthermore, the potential communication routes between the ATP-binding region and the DNA-binding region of each recombinase were identified. Our results demonstrate the conserved molecular dynamics of these recombinases in the early stage of homologous recombination, including cooperative motions between regions, conserved sensing and effecting functional residue distribution, and conserved hub residues. Meanwhile, the unique ATP-DNA communication routes of each recombinase was also revealed. These results provide new insights into the mechanism of RecA/Rad51 family proteins, and provide new theoretical guidance for the development of allosteric inhibitors and the application of RecA/Rad51 family proteins.

Divalent metal cofactors differentially modulate RadA-mediated strand invasion and exchange in Saccharolobus solfataricus

Central to the universal process of recombination, RecA family proteins form nucleoprotein filaments to catalyze production of heteroduplex DNA between substrate ssDNAs and template dsDNAs. ATP binding assists the filament in assuming the necessary conformation for forming heteroduplex DNA, but hydrolysis is not required. ATP hydrolysis has two identified roles which are not universally conserved: promotion of filament dissociation and enhancing flexibility of the filament. In this work, we examine ATP utilization of the RecA family recombinase SsoRadA from Saccharolobus solfataricus to determine its function in recombinase-mediated heteroduplex DNA formation. Wild-type SsoRadA protein and two ATPase mutant proteins were evaluated for the effects of three divalent metal cofactors. We found that unlike other archaeal RadA proteins, SsoRadA-mediated strand exchange is not enhanced by Ca2+. Instead, the S. solfataricus recombinase can utilize Mn2+ to stimulate strand invasion and reduce ADP-binding stability. Additionally, reduction of SsoRadA ATPase activity by Walker Box mutation or cofactor alteration resulted in a loss of large, complete strand exchange products. Depletion of ADP was found to improve initial strand invasion but also led to a similar loss of large strand exchange events. Our results indicate that overall, SsoRadA is distinct in its use of divalent cofactors but its activity with Mn2+ shows similarity to human RAD51 protein with Ca2+.

Role of RadA and DNA Polymerases in Recombination-Associated DNA Synthesis in Hyperthermophilic Archaea

Among the three domains of life, the process of homologous recombination (HR) plays a central role in the repair of double-strand DNA breaks and the restart of stalled replication forks. Curiously, main protein actors involved in the HR process appear to be essential for hyperthermophilic Archaea raising interesting questions about the role of HR in replication and repair strategies of those Archaea living in extreme conditions. One key actor of this process is the recombinase RadA, which allows the homologous strand search and provides a DNA substrate required for following DNA synthesis and restoring genetic information. DNA polymerase operation after the strand exchange step is unclear in Archaea. Working with Pyrococcus abyssi proteins, here we show that both DNA polymerases, family-B polymerase (PolB) and family-D polymerase (PolD), can take charge of processing the RadA-mediated recombination intermediates. Our results also indicate that PolD is far less efficient, as compared with PolB, to extend the invaded DNA at the displacement-loop (D-loop) substrate. These observations coincide with previous genetic analyses obtained on Thermococcus species showing that PolB is mainly involved in DNA repair without being essential probably because PolD could take over combined with additional partners.

Studies on DNA-related enzymes to elucidate molecular mechanisms underlying genetic information processing and their application in genetic engineering

Recombinant DNA technology, in which artificially "cut and pasted" DNA in vitro is introduced into living cells, contributed extensively to the rapid development of molecular biology over the past 5 decades since the latter half of the 20^th century. Although the original technology required special experiences and skills, the development of polymerase chain reaction (PCR) has greatly eased in vitro genetic manipulation for various experimental methods. The current development of a simple genome-editing technique using CRISPR-Cas9 gave great impetus to molecular biology. Genome editing is a major technique for elucidating the functions of many unknown genes. Genetic manipulation technologies rely on enzymes that act on DNA. It involves artificially synthesizing, cleaving, and ligating DNA strands by making good use of DNA-related enzymes present in organisms to maintain their life activities. In this review, I focus on key enzymes involved in the development of genetic manipulation technologies.

The RadA Recombinase and Paralogs of the Hyperthermophilic Archaeon Sulfolobus solfataricus

Repair of DNA double-strand breaks is a critical function shared by organisms in all three domains of life. The majority of mechanistic understanding of this process has come from characterization of bacterial and eukaryotic proteins, while significantly less is known about analogous activities in the third, archaeal domain. Despite the physical resemblance of archaea to bacteria, archaeal proteins involved in break repair are remarkably similar to those used by eukaryotes. Investigating the function of the archaeal version of these proteins is, in many cases, simpler than working with eukaryotic homologs owing to their robust nature and ease of purification. In this chapter, we describe methods for purification and activity analysis for the RadA recombinase and its paralogs from the hyperthermophilic acidophilic archaeon Sulfolobus solfataricus.

Archaeal MutS5 tightly binds to Holliday junction similarly to eukaryotic MutSγ

Archaeal DNA recombination mechanism and the related proteins are similar to those in eukaryotes. However, no functional homolog of eukaryotic MutSγ, which recognizes Holliday junction to promote homologous recombination, has been identified in archaea. Hence, the whole molecular mechanism of archaeal homologous recombination has not yet been revealed. In this study, to identify the archaeal functional homolog of MutSγ, we focused on a functionally uncharacterized MutS homolog, MutS5, from a hyperthermophilic archaeon Pyrococcus horikoshii (phMutS5). Archaeal MutS5 has a Walker ATPase motif-containing amino acid sequence that shows similarity to the ATPase domain of MutSγ. It is known that the ATPase domain of MutS homologs is also a dimerization domain. Chemical cross-linking revealed that purified phMutS5 has an ability to dimerize in solution. phMutS5 bound to Holliday junction with a higher affinity than to other branched and linear DNAs, which resembles the DNA-binding specificities of MutSγ and bacterial MutS2, a Holliday junction-resolving MutS homolog. However, phMutS5 has no nuclease activity against branched DNA unlike MutS2. The ATPase activity of phMutS5 was significantly stimulated by the presence of Holliday junction similarly to MutSγ. Furthermore, site-directed mutagenesis revealed that the ATPase activity is dependent on the Walker ATPase motif of the protein. These results suggest that archaeal MutS5 should stabilize the Holliday junction and play a role in homologous recombination, which is analogous to the function of eukaryotic MutSγ.

Protein splicing of a recombinase intein induced by ssDNA and DNA damage

In this study, Lennon et al. provide new insights into the role of inteins (or protein introns), which are known to autocatalytically excise themselves through protein splicing. They show that intein splicing can be stimulated by a substrate of the invaded host protein, suggesting a new form of post-translational control.

Inteins (or protein introns) autocatalytically excise themselves through protein splicing. We challenge the long-considered notion that inteins are merely molecular parasites and posit that some inteins evolved to regulate host protein function. Here we show substrate-induced and DNA damage-induced splicing, in which an archaeal recombinase RadA intein splices dramatically faster and more accurately when provided with ssDNA. This unprecedented example of intein splicing stimulation by the substrate of the invaded host protein provides compelling support in favor of inteins acting as pause buttons to arrest protein function until needed; then, an immediate activity switch is triggered, representing a new form of post-translational control.

Highly thermostable RadA protein from the archaeon Pyrococcus woesei enhances specificity of simplex and multiplex PCR assays

The radA gene of the hyperthermophilic archaeon Pyrococcus woesei (Thermococcales) was cloned and overexpressed in Escherichia coli. The 1050-bp gene codes for a 349-amino-acid polypeptide with an M r of 38,397 which shows 100 % positional amino acid identity to Pyrococcus furiosus RadA and 27.1 % to the E. coli RecA protein. Recombinant RadA was overproduced in Escherichia coli as a His-tagged fusion protein and purified to electrophoretic homogeneity using a simple procedure consisting of ammonium sulfate precipitation and metal-affinity chromatography. In solution RadA exists as an undecamer (11-mer). The protein binds both to ssDNA and dsDNA. RadA has been found to be highly thermostable, it remains almost unaffected by a 4-h incubation at 94 °C. The addition of the RadA protein to either simplex or multiplex PCR assays, significantly improves the specificity of DNA amplification by eliminating non-specific products. Among applications tested the RadA protein proved to be useful in allelic discrimination assay of HADHA gene associated with long-chain 3-hydroxylacyl-CoA dehydrogenase deficiency that in infancy may lead to hypotonia, serious heart and liver problems and even sudden death.

Nanobiomotors of archaeal DNA repair machineries: current research status and application potential

Nanobiomotors perform various important functions in the cell, and they also emerge as potential vehicle for drug delivery. These proteins employ conserved ATPase domains to convert chemical energy to mechanical work and motion. Several archaeal nucleic acid nanobiomotors, such as DNA helicases that unwind double-stranded DNA molecules during DNA damage repair, have been characterized in details. XPB, XPD and Hjm are SF2 family helicases, each of which employs two ATPase domains for ATP binding and hydrolysis to drive DNA unwinding. They also carry additional specific domains for substrate binding and regulation. Another helicase, HerA, forms a hexameric ring that may act as a DNA-pumping enzyme at the end processing of double-stranded DNA breaks. Common for all these nanobiomotors is that they contain ATPase domain that adopts RecA fold structure. This structure is characteristic for RecA/RadA family proteins and has been studied in great details. Here we review the structural analyses of these archaeal nucleic acid biomotors and the molecular mechanisms of how ATP binding and hydrolysis promote the conformation change that drives mechanical motion. The application potential of archaeal nanobiomotors in drug delivery has been discussed.

RecA family proteins in archaea: RadA and its cousins

Recombinases of the RecA family are essential for homologous recombination and underpin genome stability, by promoting the repair of double-stranded DNA breaks and the rescue of collapsed DNA replication forks. Until now, our understanding of homologous recombination has relied on studies of bacterial and eukaryotic model organisms. Archaea provide new opportunities to study how recombination operates in a lineage distinct from bacteria and eukaryotes. In the present paper, we focus on RadA, the archaeal RecA family recombinase, and its homologues in archaea and other domains. On the basis of phylogenetic analysis, we propose that a family of archaeal proteins with a single RecA domain, which are currently annotated as KaiC, be renamed aRadC.

The archaeal Hjm helicase has recQ-like functions, and may be involved in repair of stalled replication fork

The archaeal Hjm is a structure-specific DNA helicase, which was originally identified in the hyperthermophilic archaeon, Pyrococcus furiosus, by in vitro screening for Holliday junction migration activity. Further biochemical analyses of the Hjm protein from P. furiosus showed that this protein preferably binds to fork-related Y-structured DNAs and unwinds their double-stranded regions in vitro, just like the E. coli RecQ protein. Furthermore, genetic analyses showed that Hjm produced in E. coli cells partially complemented the defect of functions of RecQ in a recQ mutant E. coli strain. These results suggest that Hjm may be a functional counterpart of RecQ in Archaea, in which it is necessary for the maintenance of genome integrity, although the amino acid sequences are not conserved. The functional interaction of Hjm with PCNA for its helicase activity further suggests that the Hjm works at stalled replication forks, as a member of the reconstituted replisomes to restart replication.

Interactions of RadB, a DNA repair protein in archaea, with DNA and ATP

The RecA family of recombinases (RecA, Rad51, RadA and UvsX) catalyse strand-exchange between homologous DNA molecules by utilising conserved DNA-binding modules and a common core ATPase domain. RadB was identified in archaea as a Rad51-like protein on the basis of conserved ATPase sequences. However, RadB does not catalyse strand exchange and does not turn over ATP efficiently. RadB does bind DNA, and here we report a triplet of residues (Lys-His-Arg) that is highly conserved at the RadB C terminus, and is crucial for DNA binding. This is consistent with the motif forming a "basic patch" of highly conserved residues identified in an atomic structure of RadB from Thermococcus kodakaraensis. As the triplet motif is conserved at the C terminus of XRCC2 also, a mammalian Rad51-paralogue, we present a phylogenetic analysis that clarifies the relationship between RadB, Rad51-paralogues and recombinases. We investigate interactions between RadB and ATP using genetics and biochemistry; ATP binding by RadB is needed to promote survival of Haloferax volcanii after UV irradiation, and ATP, but not other NTPs, induces pronounced conformational change in RadB. This is the first genetic analysis of radB, and establishes its importance for maintaining genome stability in archaea. ATP-induced conformational change in RadB may explain previous reports that RadB controls Holliday junction resolution by Hjc, depending on the presence or the absence of ATP.

Structure of RadB recombinase from a hyperthermophilic archaeon, Thermococcus kodakaraensis KOD1: an implication for the formation of a near-7-fold helical assembly

The X-ray crystal structure of RadB from Thermococcus kodakaraensis KOD1, an archaeal homologue of the RecA/Rad51 family proteins, have been determined in two crystal forms. The structure represents the core ATPase domain of the RecA/Rad51 proteins. Two independent molecules in the type 1 crystal were roughly related by 7-fold screw symmetry whereas non-crystallographic 2-fold symmetry was observed in the type 2 crystal. The dimer structure in the type 1 crystal is extended to construct a helical assembly, which resembles the filamentous structures reported for other RecA/Rad51 proteins. The molecular interface in the type 1 dimer is formed by facing a basic surface patch of one monomer to an acidic one of the other. The empty ATP binding pocket is located at the interface and barely concealed from the outside similarly to that in the active form of the RecA filament. The model assembly has a positively charged belt on one surface bordering the helical groove suitable for facile binding of DNA. Electron microscopy has revealed that, in the absence of ATP and DNA, RadB forms a filament with a similar diameter to that of the hypothetical assembly, although its helical properties were not confirmed.

Characteristic thermodependence of the RadA recombinase from the hyperthermophilic archaeon Desulfurococcus amylolyticus

The Desulfurococcus amylolyticus RadA protein (RadA(Da)) promotes recombination at temperatures approaching the DNA melting point. Here, analyzing ATPase of the RadA(Da) presynaptic complex, we described other distinguishing characteristics of RadA(Da). These include sensitivity to NaCl, preference for lengthy single-stranded DNA as a cofactor, protein activity at temperatures of over 100 degrees C, and bimodal ATPase activity. These characteristics suggest that RadA(Da) is a founding member of a new class of archaeal recombinases.

Conformational flexibility revealed by the crystal structure of a crenarchaeal RadA

Homologous recombinational repair is an essential mechanism for repair of double-strand breaks in DNA. Recombinases of the RecA-fold family play a crucial role in this process, forming filaments that utilize ATP to mediate their interactions with single- and double-stranded DNA. The recombinase molecules present in the archaea (RadA) and eukaryota (Rad51) are more closely related to each other than to their bacterial counterpart (RecA) and, as a result, RadA makes a suitable model for the eukaryotic system. The crystal structure of Sulfolobus solfataricus RadA has been solved to a resolution of 3.2 Å in the absence of nucleotide analogues or DNA, revealing a narrow filamentous assembly with three molecules per helical turn. As observed in other RecA-family recombinases, each RadA molecule in the filament is linked to its neighbour via interactions of a short β-strand with the neighbouring ATPase domain. However, despite apparent flexibility between domains, comparison with other structures indicates conservation of a number of key interactions that introduce rigidity to the system, allowing allosteric control of the filament by interaction with ATP. Additional analysis reveals that the interaction specificity of the five human Rad51 paralogues can be predicted using a simple model based on the RadA structure.

Identification of a novel helicase activity unwinding branched DNAs from the hyperthermophilic archaeon, Pyrococcus furiosus

To identify the branch migration activity in archaea, we fractionated Pyrococcus furiosus cell extracts by several chromatography and assayed for ATP-dependent resolution of synthetic Holliday junctions. The target activity was identified in the column fractions, and the optimal reaction conditions for the branch migration activity were determined using the partially purified fraction. We successfully cloned the corresponding gene by screening a heat-stable protein library made by P. furiosus genomic DNA. The gene, hjm (Holliday junction migration), encodes a protein composed of 720 amino acids. The Hjm protein is conserved in Archaea and belongs to the helicase superfamily 2. A homology search revealed that Hjm shares sequence similarity with the human PolTheta, HEL308, and Drosophila Mus308 proteins, which are involved in a DNA repair, whereas no similar sequences were found in bacteria and yeast. The Hjm helicase may play a central role in the repair systems of organisms living in extreme environments.

Cooperation of the N-terminal Helicase and C-terminal endonuclease activities of Archaeal Hef protein in processing stalled replication forks

Blockage of replication fork progression often occurs during DNA replication, and repairing and restarting stalled replication forks are essential events in all organisms for the maintenance of genome integrity. The repair system employs processing enzymes to restore the stalled fork. In Archaea Hef is a well conserved protein that specifically cleaves nicked, flapped, and fork-structured DNAs. This enzyme contains two distinct domains that are similar to the DEAH helicase family and XPF nuclease superfamily proteins. Analyses of truncated mutant proteins consisting of each domain revealed that the C-terminal nuclease domain independently recognized and incised fork-structured DNA. The N-terminal helicase domain also specifically unwound fork-structured DNA and Holliday junction DNA in the presence of ATP. Moreover, the endonuclease activity of the whole Hef protein was clearly stimulated by ATP hydrolysis catalyzed by the N-terminal domain. These enzymatic properties suggest that Hef efficiently resolves stalled replication forks by two steps, which are branch point transfer to the 5'-end of the nascent lagging strand by the N-terminal helicase followed by template strand incision for leading strand synthesis by the C-terminal endonuclease.

Novel endonuclease in Archaea cleaving DNA with various branched structure

We identified a novel structure-specific endonuclease in Pyrococcus furiosus. This nuclease contains two distinct domains, which are similar to the DEAH helicase family at the N-terminal two-third and the XPF endonuclease superfamily at the C-terminal one-third of the protein, respectively. The C-terminal domain has an endonuclease activity cleaving the DNA strand at the 5'-side of nicked or flapped positions in the duplex DNA. The nuclease also incises in the proximity of the 5'-side of a branch point in the template strand for leading synthesis in the fork-structured DNA. The N-terminal helicase may work cooperatively to change the fork structure suitable for cleavage by the C-terminal endonuclease. This protein, designated as Hef (helicase-associated endonuclease for fork-structured DNA), may be a prototypical enzyme for resolving stalled forks during DNA replication, as well as working at nucleotide excision repair.