RPA subunit arrangement near the 3'-end of the primer is modulated by the length of the template strand and cooperative protein interactions

ssDNA accessibility of Rad51 is regulated by orchestrating multiple RPA dynamics

The eukaryotic single-stranded DNA (ssDNA)-binding protein Replication Protein A (RPA) plays a crucial role in various DNA metabolic pathways, including DNA replication and repair, by dynamically associating with ssDNA. While the binding of a single RPA molecule to ssDNA has been thoroughly studied, the accessibility of ssDNA is largely governed by the bimolecular behavior of RPA, the biophysical nature of which remains unclear. In this study, we develop a three-step low-complexity ssDNA Curtains method, which, when combined with biochemical assays and a Markov chain model in non-equilibrium physics, allow us to decipher the dynamics of multiple RPA binding to long ssDNA. Interestingly, our results suggest that Rad52, the mediator protein, can modulate the ssDNA accessibility of Rad51, which is nucleated on RPA coated ssDNA through dynamic ssDNA exposure between neighboring RPA molecules. We find that this process is controlled by the shifting between the protection mode and action mode of RPA ssDNA binding, where tighter RPA spacing and lower ssDNA accessibility are favored under RPA protection mode, which can be facilitated by the Rfa2 WH domain and inhibited by Rad52 RPA interaction.

Interdomain dynamics in human Replication Protein A regulates kinetics and thermodynamics of its binding to ssDNA

Human Replication Protein A (hRPA) is a multidomain protein that interacts with ssDNA intermediates to provide the latter much-needed stability during DNA metabolism and maintain genomic integrity. Although the ssDNA organization with hRPA was studied recently through experimental means, characterizing the underlying mechanism at the atomic level remains challenging because of the dynamic domain architecture of hRPA and poorly understood heterogeneity of ssDNA-protein interactions. Here, we used a computational framework, precisely tailored to capture protein-ssDNA interactions, and investigated the binding of hRPA with a 60 nt ssDNA. Two distinct binding mechanisms are realized based on the hRPA domain flexibility. For a rigid domain architecture of hRPA, ssDNA binds sequentially with hRPA domains, resulting in slow association kinetics. The binding pathway involves the formation of stable and distinct intermediate states. On contrary, for a flexible domain architecture of hRPA, ssDNA binds synergistically to the A and B domains followed by the rest of hRPA. The domain dynamics in hRPA alleviates the free energy cost of domain orientation necessary for specific binding with ssDNA, leading to fast association kinetics along a downhill binding free energy landscape. An ensemble of free energetically degenerate intermediate states is encountered that makes it arduous to characterize them structurally. An excellent match between our results with the available experimental observations provides new insights into the rich dynamics of hRPA binding to ssDNA and in general paves the way to investigate intricate details of ssDNA-protein interactions, crucial for cellular functioning.

Strand Displacement Activity of PrimPol

Human PrimPol is a unique enzyme possessing DNA/RNA primase and DNA polymerase activities. In this work, we demonstrated that PrimPol efficiently fills a 5-nt gap and possesses the conditional strand displacement activity stimulated by Mn²⁺ ions and accessory replicative proteins RPA and PolDIP2. The DNA displacement activity of PrimPol was found to be more efficient than the RNA displacement activity and FEN1 processed the 5′-DNA flaps generated by PrimPol in vitro.

Replication protein A: a multifunctional protein with roles in DNA replication, repair and beyond

Single-stranded DNA (ssDNA) forms continuously during DNA replication and is an important intermediate during recombination-mediated repair of damaged DNA. Replication protein A (RPA) is the major eukaryotic ssDNA-binding protein. As such, RPA protects the transiently formed ssDNA from nucleolytic degradation and serves as a physical platform for the recruitment of DNA damage response factors. Prominent and well-studied RPA-interacting partners are the tumor suppressor protein p53, the RAD51 recombinase and the ATR-interacting proteins ATRIP and ETAA1. RPA interactions are also documented with the helicases BLM, WRN and SMARCAL1/HARP, as well as the nucleotide excision repair proteins XPA, XPG and XPF–ERCC1. Besides its well-studied roles in DNA replication (restart) and repair, accumulating evidence shows that RPA is engaged in DNA activities in a broader biological context, including nucleosome assembly on nascent chromatin, regulation of gene expression, telomere maintenance and numerous other aspects of nucleic acid metabolism. In addition, novel RPA inhibitors show promising effects in cancer treatment, as single agents or in combination with chemotherapeutics. Since the biochemical properties of RPA and its roles in DNA repair have been extensively reviewed, here we focus on recent discoveries describing several non-canonical functions.

Photoreactive DNA as a Tool to Study Replication Protein A Functioning in DNA Replication and Repair

Replication protein A (RPA), eukaryotic single-stranded DNA-binding protein, is a key player in multiple processes of DNA metabolism including DNA replication, recombination and DNA repair. Human RPA composed of subunits of 70-, 32- and 14-kDa binds ssDNA with high affinity and interacts specifically with multiple proteins. The RPA heterotrimer binds ssDNA in several modes, with occlusion lengths of 8-10, 13-22 and 30 nucleotides corresponding to global, transitional and elongated conformations of protein. Varying the structure of photoreactive DNA, the intermediates of different stages of DNA replication or DNA repair were designed and applied to identify positioning of the RPA subunits on the specific DNA structures. Using this approach, RPA interactions with various types of DNA structures attributed to replication and DNA repair intermediates were examined. This review is dedicated to blessed memory of Prof. Alain Favre who contributed to the development of photoreactive nucleotide derivatives and their application for the study of protein-nucleic acids interactions.

Human replication protein A induces dynamic changes in single-stranded DNA and RNA structures

Replication protein A (RPA) is the major eukaryotic ssDNA-binding protein and has essential roles in genome maintenance. RPA binds to ssDNA through multiple modes, and recent studies have suggested that the RPA-ssDNA interaction is dynamic. However, how RPA alternates between different binding modes and modifies ssDNA structures in this dynamic interaction remains unknown. Here, we used single-molecule FRET to systematically investigate the interaction between human RPA and ssDNA. We show that RPA can adopt different types of binding complexes with ssDNAs of different lengths, leading to the straightening or bending of the ssDNAs, depending on both the length and structure of the ssDNA substrate and the RPA concentration. Importantly, we noted that some of the complexes are highly dynamic, whereas others appear relatively static. On the basis of the above observations, we propose a model explaining how RPA dynamically engages with ssDNA. Of note, fluorescence anisotropy indicated that RPA can also associate with RNA but with a lower binding affinity than with ssDNA. At the single-molecule level, we observed that RPA is undergoing rapid and repetitive associations with and dissociation from the RNA. This study may provide new insights into the rich dynamics of RPA binding to ssDNA and RNA.

Replication protein A complex in Thermococcus kodakarensis interacts with DNA polymerases and helps their effective strand synthesis

Replication protein A (RPA) is an essential component of DNA metabolic processes. RPA binds to single-stranded DNA (ssDNA) and interacts with multiple DNA-binding proteins. In this study, we showed that two DNA polymerases, PolB and PolD, from the hyperthermophilic archaeon Thermococcus kodakarensis interact directly with RPA in vitro. RPA was expected to play a role in resolving the secondary structure, which may stop the DNA synthesis reaction, in the template ssDNA. Our in vitro DNA synthesis assay showed that the pausing was resolved by RPA for both PolB and PolD. These results supported the fact that RPA interacts with DNA polymerases as a member of the replisome and is involved in the normal progression of DNA replication forks.

Replication protein A as a modulator of the poly(ADP-ribose)polymerase 1 activity

Replication protein A contributes to all major pathways of DNA metabolism and is a target for post-translation modifications, including poly(ADP-ribosyl)ation catalyzed by PARP1. Here we demonstrate that the efficiency of RPA poly(ADP-ribosyl)ation strongly depends on the structure of DNA used for PARP1 activation and on the polarity of RPA binding. Moreover, RPA influences PARP1 activity, and this effect also depends on DNA structure: RPA inhibits PAR synthesis catalyzed by PARP1 in the presence of ssDNA and stimulates it in the presence of a DNA duplex, in particular that containing a nick or a gap. Using fluorescently labeled proteins, we showed their direct interaction and characterized it quantitatively. RPA can accelerate the replacement of poly(ADP-ribosyl)ated PARP1 molecules bound to DNA by the unmodified ones. Thus, our data allow us to suggest that the balance between the affinities of PARP1 and RPA for DNA and the interaction of these proteins with each other are the cornerstone of the modulating effect of RPA on PARP1 activity. This effect might contribute to the regulation of PARP1 activity in various DNA processing mechanisms including DNA replication and repair pathways, where both PARP1 and RPA participate.

RPA and XPA interaction with DNA structures mimicking intermediates of the late stages in nucleotide excision repair

Replication protein A (RPA) and the xeroderma pigmentosum group A (XPA) protein are indispensable for both pathways of nucleotide excision repair (NER). Here we analyze the interaction of RPA and XPA with DNA containing a flap and different size gaps that imitate intermediates of the late NER stages. Using gel mobility shift assays, we found that RPA affinity for DNA decreased when DNA contained both extended gap and similar sized flap in comparison with gapped-DNA structure. Moreover, crosslinking experiments with the flap-gap DNA revealed that RPA interacts mainly with the ssDNA platform within the long gap and contacts flap in DNA with a short gap. XPA exhibits higher affinity for bubble-DNA structures than to flap-gap-containing DNA. Protein titration analysis showed that formation of the RPA-XPA-DNA ternary complex depends on the protein concentration ratio and these proteins can function as independent players or in tandem. Using fluorescently-labelled RPA, direct interaction of this protein with XPA was detected and characterized quantitatively. The data obtained allow us to suggest that XPA can be involved in the post-incision NER stages via its interaction with RPA.

5' to 3' Unfolding Directionality of DNA Secondary Structures by Replication Protein A: G-QUADRUPLEXES AND DUPLEXES

The replication protein A (RPA) is a single-stranded DNA-binding protein that plays an essential role in DNA metabolism. RPA is able to unfold G-quadruplex (G4) structures formed by telomeric DNA sequences, a function important for telomere maintenance. To elucidate the mechanism through which RPA unfolds telomeric G4s, we studied its interaction with oligonucleotides that adopt a G4 structure extended with a single-stranded tail on either side of the G4. Binding and unfolding was characterized using several biochemical and biophysical approaches and in the presence of specific G4 ligands, such as telomestatin and 360A. Our data show that RPA can bind on each side of the G4 but it unwinds the G4 only from 5' toward 3'. We explain the 5' to 3' unfolding directionality in terms of the 5' to 3' oriented laying out of hRPA subunits along single-stranded DNA. Furthermore, we demonstrate by kinetics experiments that RPA proceeds with the same directionality for duplex unfolding.

High-affinity DNA-binding domains of replication protein A (RPA) direct SMARCAL1-dependent replication fork remodeling

SMARCAL1 catalyzes replication fork remodeling to maintain genome stability. It is recruited to replication forks via an interaction with replication protein A (RPA), the major ssDNA-binding protein in eukaryotic cells. In addition to directing its localization, RPA also activates SMARCAL1 on some fork substrates but inhibits it on others, thereby conferring substrate specificity to SMARCAL1 fork-remodeling reactions. We investigated the mechanism by which RPA regulates SMARCAL1. Our results indicate that although an interaction between SMARCAL1 and RPA is essential for SMARCAL1 activation, the location of the interacting surface on RPA is not. Counterintuitively, high-affinity DNA binding of RPA DNA-binding domain (DBD) A and DBD-B near the fork junction makes it easier for SMARCAL1 to remodel the fork, which requires removing RPA. We also found that RPA DBD-C and DBD-D are not required for SMARCAL1 regulation. Thus, the orientation of the high-affinity RPA DBDs at forks dictates SMARCAL1 substrate specificity.

A new structural framework for integrating replication protein A into DNA processing machinery

By coupling the protection and organization of single-stranded DNA (ssDNA) with recruitment and alignment of DNA processing factors, replication protein A (RPA) lies at the heart of dynamic multi-protein DNA processing machinery. Nevertheless, how RPA coordinates biochemical functions of its eight domains remains unknown. We examined the structural biochemistry of RPA’s DNA-binding activity, combining small-angle X-ray and neutron scattering with all-atom molecular dynamics simulations to investigate the architecture of RPA’s DNA-binding core. The scattering data reveal compaction promoted by DNA binding; DNA-free RPA exists in an ensemble of states with inter-domain mobility and becomes progressively more condensed and less dynamic on binding ssDNA. Our results contrast with previous models proposing RPA initially binds ssDNA in a condensed state and becomes more extended as it fully engages the substrate. Moreover, the consensus view that RPA engages ssDNA in initial, intermediate and final stages conflicts with our data revealing that RPA undergoes two (not three) transitions as it binds ssDNA with no evidence for a discrete intermediate state. These results form a framework for understanding how RPA integrates the ssDNA substrate into DNA processing machinery, provides substrate access to its binding partners and promotes the progression and selection of DNA processing pathways.

Telomere protection by TPP1/POT1 requires tethering to TIN2

To prevent ATR activation, telomeres deploy the single-stranded DNA binding activity of TPP1/POT1a. POT1a blocks the binding of RPA to telomeres, suggesting that ATR is repressed through RPA exclusion. However, comparison of the DNA binding affinities and abundance of TPP1/POT1a and RPA indicates that TPP1/POT1a by itself is unlikely to exclude RPA. We therefore analyzed the central shelterin protein TIN2, which links TPP1/POT1a (and POT1b) to TRF1 and TRF2 on the double-stranded telomeric DNA. Upon TIN2 deletion, telomeres lost TPP1/POT1a, accumulated RPA, elicited an ATR signal, and showed all other phenotypes of POT1a/b deletion. TIN2 also affected the TRF2-dependent repression of ATM kinase signaling but not to TRF2-mediated inhibition of telomere fusions. Thus, while TIN2 has a minor contribution to the repression of ATM by TRF2, its major role is to stabilize TPP1/POT1a on the ss telomeric DNA, thereby allowing effective exclusion of RPA and repression of ATR signaling.

Accurate prediction of the binding free energy and analysis of the mechanism of the interaction of replication protein A (RPA) with ssDNA

The eukaryotic replication protein A (RPA) has several pivotal functions in the cell metabolism, such as chromosomal replication, prevention of hairpin formation, DNA repair and recombination, and signaling after DNA damage. Moreover, RPA seems to have a crucial role in organizing the sequential assembly of DNA processing proteins along single stranded DNA (ssDNA). The strong RPA affinity for ssDNA, K(A) between 10(-9)-10(-10) M, is characterized by a low cooperativity with minor variation for changes on the nucleotide sequence. Recently, new data on RPA interactions was reported, including the binding free energy of the complex RPA70AB with dC(8) and dC(5), which has been estimated to be -10 ± 0.4 kcal mol(-1) and -7 ± 1 kcal mol(-1), respectively. In view of these results we performed a study based on molecular dynamics aimed to reproduce the absolute binding free energy of RPA70AB with the dC(5) and dC(8) oligonucleotides. We used several tools to analyze the binding free energy, rigidity, and time evolution of the complex. The results obtained by MM-PBSA method, with the use of ligand free geometry as a reference for the receptor in the separate trajectory approach, are in excellent agreement with the experimental data, with ±4 kcal mol(-1) error. This result shows that the MM-PB(GB)SA methods can provide accurate quantitative estimates of the binding free energy for interacting complexes when appropriate geometries are used for the receptor, ligand and complex. The decomposition of the MM-GBSA energy for each residue in the receptor allowed us to correlate the change of the affinity of the mutated protein with the ΔG(gas+sol) contribution of the residue considered in the mutation. The agreement with experiment is optimal and a strong change in the binding free energy can be considered as the dominant factor in the loss for the binding affinity resulting from mutation.

Telomere protection by TPP1/POT1 requires tethering to TIN2

To prevent ATR activation, telomeres deploy the single-stranded DNA binding activity of TPP1/POT1a. POT1a blocks the binding of RPA to telomeres, suggesting that ATR is repressed through RPA exclusion. However, comparison of the DNA binding affinities and abundance of TPP1/POT1a and RPA indicates that TPP1/POT1a by itself is unlikely to exclude RPA. We therefore analyzed the central shelterin protein TIN2, which links TPP1/POT1a (and POT1b) to TRF1 and TRF2 on the double-stranded telomeric DNA. Upon TIN2 deletion, telomeres lost TPP1/POT1a, accumulated RPA, elicited an ATR signal, and showed all other phenotypes of POT1a/b deletion. TIN2 also affected the TRF2-dependent repression of ATM kinase signaling but not to TRF2-mediated inhibition of telomere fusions. Thus, while TIN2 has a minor contribution to the repression of ATM by TRF2, its major role is to stabilize TPP1/POT1a on the ss telomeric DNA, thereby allowing effective exclusion of RPA and repression of ATR signaling.

Nucleotide excision repair: DNA damage recognition and preincision complex assembly

Nucleotide excision repair (NER) is one of the major DNA repair pathways in eukaryotic cells counteracting genetic changes caused by DNA damage. NER removes a wide set of structurally diverse lesions such as pyrimidine dimers arising upon UV irradiation and bulky chemical adducts arising upon exposure to carcinogens or chemotherapeutic drugs. NER defects lead to severe diseases including some forms of cancer. In view of the broad substrate specificity of NER, it is of interest to understand how a certain set of proteins recognizes various DNA lesions in the context of a large excess of intact DNA. This review focuses on DNA damage recognition and following stages resulting in preincision complex assembly, the key and still most unclear steps of NER. The major models of primary damage recognition and preincision complex assembly are considered. The contribution of affinity labeling techniques in study of this process is discussed.

Localization of xeroderma pigmentosum group A protein and replication protein A on damaged DNA in nucleotide excision repair

The interaction of xeroderma pigmentosum group A protein (XPA) and replication protein A (RPA) with damaged DNA in nucleotide excision repair (NER) was studied using model dsDNA and bubble-DNA structure with 5-{3-[6-(carboxyamido-fluoresceinyl)amidocapromoyl]allyl}-dUMP lesions in one strand and containing photoreactive 5-iodo-dUMP residues in defined positions. Interactions of XPA and RPA with damaged and undamaged DNA strands were investigated by DNA–protein photocrosslinking and gel shift analysis. XPA showed two maximums of crosslinking intensities located on the 5′-side from a lesion. RPA mainly localized on undamaged strand of damaged DNA duplex and damaged bubble-DNA structure. These results presented for the first time the direct evidence for the localization of XPA in the 5′-side of the lesion and suggested the key role of XPA orientation in conjunction with RPA binding to undamaged strand for the positioning of the NER preincision complex. The findings supported the mechanism of loading of the heterodimer consisting of excision repair cross-complementing group 1 and xeroderma pigmentosum group F proteins by XPA on the 5′-side from the lesion before damaged strand incision. Importantly, the proper orientation of XPA and RPA in the stage of preincision was achieved in the absence of TFIIH and XPG.

Secondary structure formation and DNA instability at fragile site FRA16B

Human chromosomal fragile sites are specific loci that are especially susceptible to DNA breakage following conditions of partial replication stress. They often are found in genes involved in tumorigenesis and map to over half of all known cancer-specific recurrent translocation breakpoints. While their molecular basis remains elusive, most fragile DNAs contain AT-rich flexibility islands predicted to form stable secondary structures. To understand the mechanism of fragile site instability, we examined the contribution of secondary structure formation to breakage at FRA16B. Here, we show that FRA16B forms an alternative DNA structure in vitro. During replication in human cells, FRA16B exhibited reduced replication efficiency and expansions and deletions, depending on replication orientation and distance from the origin. Furthermore, the examination of a FRA16B replication fork template demonstrated that the majority of the constructs contained DNA polymerase paused within the FRA16B sequence, and among the molecules, which completed DNA synthesis, 81% of them underwent fork reversal. These results strongly suggest that the secondary-structure-forming ability of FRA16B contributes to its fragility by stalling DNA replication, and this mechanism may be shared among other fragile DNAs.

Nucleotide excision repair in higher eukaryotes: mechanism of primary damage recognition in global genome repair

Nucleotide excision repair (NER) is one of the major DNA repair pathways in eukaryotic cells that counteract the formation of genetic damage. NER removes structurally diverse lesions such as pyrimidine dimers, arising upon UV irradiation, and bulky chemical adducts, arising upon exposure to carcinogens and some chemotherapeutic drugs. NER defects lead to severe diseases, including some forms of cancer. In view of the broad substrate specificity of NER, it is of interest to understand how a certain set of proteins recognizes various DNA lesions in the contest of a large excess of intact DNA. This review focuses on DNA damage recognition, the key and, as yet, most questionable step of NER. Understanding of mechanism of this step of NER may give a key contribution to study of similar processes of DNA damage recognition (base excision repair, mismatch repair) and regulation of assembly of various DNA repair machines. The major models of primary damage recognition and pre-incision complex assembly are considered. The model of a sequential loading of repair proteins on damaged DNA seems most reasonable in the light of the available data. The possible contribution of affinity labeling technique in study of this process is discussed.

Mechanisms of single-stranded DNA-binding protein functioning in cellular DNA metabolism

This review deals with analysis of mechanisms involved in coordination of DNA replication and repair by SSB proteins; characteristics of eukaryotic, prokaryotic, and archaeal SSB proteins are considered, which made it possible to distinguish general mechanisms specific for functioning of proteins from organisms of different life domains. Mechanisms of SSB protein interactions with DNA during metabolism of the latter are studied; structural organization of the SSB protein complexes with DNA, as well as structural and functional peculiarities of different SSB proteins are analyzed.

Replication protein A modulates the activity of human telomerase in vitro

Our aim was to investigate how replication protein A (RPA) in a wide range of concentration can regulate the activity of human telomerase. We used an in vitro system based on human cell extracts with or without RPA. It has been shown that removal of RPA leads to loss of telomerase activity and addition of RPA restores telomerase activity and at the same time promotes telomerase processivity. However, high excess of RPA inhibited telomerase processivity and caused the synthesis of relatively short DNA fragments (about 50-100 nucleotides). We assume that, together with other telomere-binding proteins, RPA may take part in activation of telomere overhang elongation by telomerase at a certain stage of a cell cycle as well as in regulation of telomere length.

Circular dichroism spectra and electrophoretic mobility shift assays show that human replication protein A binds and melts intramolecular G-quadruplex structures

Noncanonical DNA structures such as G-quadruplexes might obstruct the binding of hRPA, compromising the accuracy of replication, and be a source of genomic instability. In this study, circular dichroism (CD) and electrophoretic mobility shift assay (EMSA) experiments were used to show that hRPA can bind and melt nontelomeric, intramolecular DNA G-quadruplexes under physiologically germane conditions. EMSA results show that hRPA binds to a 58-mer that includes an embedded quadruplex with an affinity equal to or greater than to nonquadruplex forming 58-mers. Moreover, hRPA binds to a 26-mer purine-rich quadruplex-forming sequence with an affinity indistinguishable from that for binding to the complementary pyrimidine-rich sequence. Under the same conditions, hRPA does not have significant affinity for binding to the duplex formed from the two sequences. Thus, DNA secondary structures can significantly modulate the binding affinity of hRPA over and above its known preference for pyrimidine-rich single-stranded sequences, so that at least some intramolecular G-quadruplex structures may not inhibit hRPA binding during DNA replication. CD spectral changes in combination with EMSA titrations suggest that one hRPA heterotrimer is sufficient to form a stable complex with an unfolded 26-mer G-quadruplex prior to the binding of a second hRPA molecule.

Essential functions of the 32 kDa subunit of yeast replication protein A

Replication protein A (RPA) is a heterotrimeric (70, 32 and 14 kDa subunits), single-stranded DNA-binding protein required for cellular DNA metabolism. All subunits of RPA are essential for life, but the specific functions of the 32 and 14 kDa subunits remains unknown. The 32 kDa subunit (RPA2) has multiple domains, but only the central DNA-binding domain (called DBD D) is essential for life in Saccharomyces cerevisiae. To define the essential function(s) of RPA2 in S. cerevisiae, a series of site-directed mutant forms of DBD D were generated. These mutant constructs were then characterized in vitro and in vivo. The mutations had minimal effects on the overall structure and activity of the RPA complex. However, several mutants were shown to disrupt crosslinking of RPA2 to DNA and to dramatically lower the DNA-binding affinity of a RPA2-containing subcomplex. When introduced into S. cerevisiae, all DBD D mutants were viable and supported normal growth rates and DNA replication. These findings indicate that RPA2–DNA interactions are not essential for viability and growth in S. cerevisiae. We conclude that DNA-binding activity of RPA2 is dispensable in yeast and that the essential function of DBD D is intra- and/or inter-protein interactions.

Evidence for direct contact between the RPA3 subunit of the human replication protein A and single-stranded DNA

Replication Protein A is a single-stranded (ss) DNA-binding protein that is highly conserved in eukaryotes and plays essential roles in many aspects of nucleic acid metabolism, including replication, recombination, DNA repair and telomere maintenance. It is a heterotrimeric complex consisting of three subunits: RPA1, RPA2 and RPA3. It possesses four DNA-binding domains (DBD), DBD-A, DBD-B and DBD-C in RPA1 and DBD-D in RPA2, and it binds ssDNA via a multistep pathway. Unlike the RPA1 and RPA2 subunits, no ssDNA-RPA3 interaction has as yet been observed although RPA3 contains a structural motif found in the other DBDs. We show here using 4-thiothymine residues as photoaffinity probe that RPA3 interacts directly with ssDNA on the 3'-side on a 31 nt ssDNA.

RPA repair recognition of DNA containing pyrimidines bearing bulky adducts

Recognition of new DNA nucleotide excision repair (NER) substrate analogs, 48-mer ddsDNA (damaged double-stranded DNA), by human replication protein A (hRPA) has been analyzed using fluorescence spectroscopy and photoaffinity modification. The aim of the present work was to find quantitative characteristics of RPA-ddsDNA interaction and RPA subunits role in this process. The designed DNA structures bear bulky substituted pyrimidine nitrogen bases at the inner positions of duplex forming DNA chains. The photoreactive 4-azido-2,5-difluoro-3- pyridin-6-yl (FAP) and fluorescent antracenyl, pyrenyl (Antr, Pyr) groups were introduced via different linker fragments into exo-4N of deoxycytidine or 5C of deoxyuridine. J-dU-containing DNA was used as a photoactive model of undamaged DNA strands. The reporter group was a fluorescein residue, introduced into the 5'-phosphate end of one duplex-forming DNA strand. RPA-dsDNA association constants and the molar RPA/dsDNA ratio have been calculated based on fluorescence anisotropy measurements under conditions of a 1:1 RPA/dsDNA molar ratio in complexes. The evident preference for RPA binding to ddsDNA over undamaged dsDNA distinctly depends on the adduct type and varies in the following way: undamaged dsDNA < Antr-dC-ddsDNA < mmdsDNA < FAPdU-, Pyr-dU-ddsDNA < FAP-dC-ddsDNA (K(D) = 68 +/- 1; 25 +/- 6; 13 +/- 1; 8 +/- 2, and 3.5 +/- 0.5 nM correspondingly) but weakly depends on the chain integrity. Interestingly the bulkier lesions not in all cases have a greater effect on RPA affinity to ddsDNA. The experiments on photoaffinity modification demonstrated only p70 of compactly arranged RPA directly interacting with dsDNA. The formation of RPA-ddsDNA covalent adducts was drastically reduced when both strands of DNA duplex contained virtually opposite located FAP-dC and Antr-dC. Thus RPA requires undamaged DNA strand presence for the effective interaction with dsDNA bearing bulky damages and demonstrates the early NER factors characteristic features underlying strand discrimination capacity and poor activity of the NER system toward double damaged DNA.

RPA nucleic acid-binding properties of IFI16-HIN200

InterFeron-gamma Inducible protein 16 (IFI16) belongs to the interferon inducible HIN200 protein family that contains transcriptional regulators linked to cell cycle regulation and differentiation. All family members contain at most two domains of 200 amino acids, called HIN200, each containing two Oligonucleotide/Oligosaccharide Binding (OB) folds. IFI16 is involved in transcriptional repression and is a component of the DNA repair multi-protein complex known as BASC, which forms after UV-induced DNA damage. In this study, we used fold recognition and biophysical approaches as a tool to infer and validate functions to the HIN200 domain. Since the best template to model IFI16-HIN200 is Replication Protein A (RPA) in complex with single-stranded nucleic acids, we tested six RPA nucleic acid-binding characteristics for IFI16-HIN200. Our results indicate that IFI16-HIN200 is an RPA-like, OB-fold, nucleic acid-binding protein that binds to ssDNA with higher affinity than to dsDNA, recognizes ssDNA in the same orientation as RPA, oligomerizes upon ssDNA binding, wraps and stretches ssDNA, but does not destabilize dsDNA. We finally propose a framework model explaining how the HIN200 domain could prevent ssDNA from re-annealing.

Human CyP33 binds specifically to mRNA and binding stimulates PPIase activity of hCyP33

Human nuclear cyclophilin 33 (hCyP33) was the first protein which was found to contain an RNA-binding motif and a PPIase domain. It was not known what cellular and physiological roles are played by the RNA-binding activity as well as the PPIase activity of hCyP33. In this paper, we investigated the binding specificity of hCyP33 to different cellular RNA using ion-exchange chromatography and affinity adsorption. Furthermore, the influence of different cellular RNAs to the PPIase activity of hCyP33 was investigated using a protease-coupled method. The results show that hCyP33 binds specifically to mRNA, namely poly(A)(+)RNA, and that binding stimulates the PPIase activity of hCyP33.

Structural characterization of human RPA sequential binding to single-stranded DNA using ssDNA as a molecular ruler

Human replication protein A (RPA), a heterotrimer composed of RPA70, RPA32, and RPA14 subunits, contains four single-stranded DNA (ssDNA) binding domains (DBD): DBD-A, DBD-B, and DBD-C in RPA70 and DBD-D in RPA32. Although crystallographic or NMR structures of these DBDs and a trimerization core have been determined, the structure of the full length of RPA or the RPA-ssDNA complex remains unknown. In this article, we have examined the structural features of RPA interaction with ssDNA by fluorescence spectroscopy. Using a set of oligonucleotides (dT) with varying lengths as a molecular ruler and also as the substrate, we have determined at single-nucleotide resolution the relative positions of the ssDNA with interacting intrinsic tryptophans of RPA. Our results revealed that Trp528 in DBD-C and Trp107 in DBD-D contact ssDNA at the 16th and 24th nucleotides (nt) from the 5'-end of the substrate, respectively. Evaluation of the relative spatial arrangement of RPA domains in the RPA-ssDNA complex suggested that DBD-B and DBD-C are spaced by about 4 nt ( approximately 19 A) apart, whereas DBD-C and DBD-D are spaced by about 7 nt ( approximately 34 A). On the basis of these geometric constraints, a global structure model for the binding of the major RPA DBDs to ssDNA was proposed.

Photoaffinity labeling and its application in structural biology

This review contains a brief consideration of some theoretical aspects of photoaffinity (photoreactive) labeling (PAL), and the most widely used photoreactive groups, such as arylazide, benzophenone, and 3-(trifluoromethyl)-3-phenyldiazirine, are characterized in comparison. Experimental methodology is described, including modern approaches of mass spectrometry for analysis of cross-linking products between the photoreactive probes and biomolecules. Examples of PAL application in diverse fields of structural biology during the last five-ten years are presented. Potential drug targets, transport processes, stereochemistry of interaction of G-protein-coupled receptors with ligands, as well as structural changes in nicotinic acetylcholine receptor are considered. Applications of photoaffinity ganglioside and phospholipid probes for studying biological membranes and of nucleotide probes in investigations of replicative and transcriptional complexes, as well as photoaffinity glycoconjugates for detecting carbohydrate-binding proteins are covered. In combination with modern techniques of instrumental analysis and computer-aided modeling, PAL remains the most important approach in studies on the organization of biological systems.

Human replication protein A unfolds telomeric G-quadruplexes

G-quadruplex structures inhibit telomerase activity and must be disrupted for telomere elongation during S phase. It has been suggested that the replication protein A (RPA) could unwind and maintain single-stranded DNA in a state amenable to the binding of telomeric components. We show here that under near-physiological in vitro conditions, human RPA is able to bind and unfold G-quadruplex structures formed from a 21mer human telomeric sequence. Analyses by native gel electrophoresis, cross-linking and fluorescence resonance energy transfer indicate the formation of both 1:1 and 2:1 complexes in which G-quadruplexes are unfolded. In addition, quadruplex opening by hRPA is much faster than observed with the complementary DNA, demonstrating that this protein efficiently unfolds G-quartets. A two-step mechanism accounting for the binding of hRPA to G-quadruplexes is proposed. These data point to the involvement of hRPA in regulation of telomere maintenance.

A dynamic model for replication protein A (RPA) function in DNA processing pathways

Processing of DNA in replication, repair and recombination pathways in cells of all organisms requires the participation of at least one major single-stranded DNA (ssDNA)-binding protein. This protein protects ssDNA from nucleolytic damage, prevents hairpin formation and blocks DNA reannealing until the processing pathway is successfully completed. Many ssDNA-binding proteins interact physically and functionally with a variety of other DNA processing proteins. These interactions are thought to temporally order and guide the parade of proteins that 'trade places' on the ssDNA, a model known as 'hand-off', as the processing pathway progresses. How this hand-off mechanism works remains poorly understood. Recent studies of the conserved eukaryotic ssDNA-binding protein replication protein A (RPA) suggest a novel mechanism by which proteins may trade places on ssDNA by binding to RPA and mediating conformation changes that alter the ssDNA-binding properties of RPA. This article reviews the structure and function of RPA, summarizes recent studies of RPA in DNA replication and other DNA processing pathways, and proposes a general model for the role of RPA in protein-mediated hand-off.

Mass spectrometry of UV-cross-linked protein-nucleic acid complexes: identification of amino acid residues in the single-stranded DNA-binding domain of human replication protein A

Photochemical cross-linking of human replication protein A (hRPA) to oligonucleotide dT30 was performed to enable identification of amino acid sequences that reside in the DNA-binding domain. A nucleoprotein complex, with a 1:1 protein/DNA stoichiometry, was separated from unreacted enzyme and oligonucleotide by SDS-polyacrylamide gel electrophoresis and subjected to in-gel digestion with trypsin. Three cross-linked tryptic peptides (nucleopeptides) of hRPA70xdT30 (T43, T28/29, and a truncated T24/25) were isolated. Combined mass spectrometric and C-terminal proteolysis experiments showed that at least one amino acid in the segment 235-ATAFNE-240 (located in T24/25), at least one out of the two residues sequence 269-FT-270 (located in T28/29), and at least one from the sequence 383-VSDF-386 (located in T43) were involved in cross-linking. These peptides contained aromatic residues (F238, F269, and F386 respectively) that can form base-stacking interactions with the DNA and were, therefore, most likely to be involved in cross-linking. The results obtained in this study demonstrate that a combination of exhaustive proteolysis and MALDI TOF MS can localize the sites of DNA binding to very short sequences of amino acids. Data so acquired can confirm or amend information obtained from site-directed mutagenesis and X-ray crystallography.

Phosphorylation of replication protein A by S-phase checkpoint kinases

The major eukaryotic single-stranded DNA (ssDNA) binding protein, replication protein A (RPA), is a heterotrimer with subunits of 70, 32 and 14 kDa (RPA70, RPA32 and RPA14). RPA-coated ssDNA has been implicated as one of the triggers for intra-S-phase checkpoint activation. Phosphorylation of RPA occurs in cells with damaged DNA or stalled replication forks. Here we show that human RPA70 and RPA32 can be phosphorylated by purified S-phase checkpoint kinases, ATR and Chk1. While ATR phosphorylates the N-terminus of RPA70, Chk1 preferentially phosphorylates RPA's major ssDNA binding domain. Chk1 phosphorylated RPA70 shows reduced ssDNA binding activity, and binding of RPA to ssDNA blocks Chk1 phosphorylation, suggesting that Chk1 and ssDNA compete for RPA's major ssDNA binding domain. ssDNA stimulates RPA32 phosphorylation by ATR in a length dependent manner. Furthermore, 3'-, but not 5'-, recessed single strand/double strand DNA junctions produce an even stronger stimulatory effect on RPA32 phosphorylation by ATR. This stimulation occurs for both RNA and DNA recessed ends. RPA's DNA binding polarity and its interaction to 3'-primer-template junctions contribute to efficient RPA32 phosphorylation. Progression of DNA polymerase is able to block the accessibility of the 3'-recessed ends and prevent the stimulatory effects of primer-template junctions on RPA phosphorylation by ATR. We propose models for the role of RPA phosphorylation by Chk1 in S-phase checkpoint pathways, and the possible regulation of ATR activity by different nucleic acid structures.

Modulation of replication protein A function by its hyperphosphorylation-induced conformational change involving DNA binding domain B

Human replication protein A (RPA), composed of RPA70, RPA32, and RPA14 subunits, undergoes hyperphosphorylation in cells in response to DNA damage. Hyperphosphorylation that occurs predominately in the N-terminal region of RPA32 is believed to play a role in modulating the cellular activities of RPA essential for almost all DNA metabolic pathways. To understand how the hyperphosphorylation modulates the functions of RPA, we compared the structural characteristics of full-length native and hyperphosphorylated RPAs using mass spectrometric protein footprinting, fluorescence spectroscopy, and limited proteolysis. Our mass spectrometric data showed that of 24 lysines and 18 arginines readily susceptible to small chemical reagent modification in native RPA, the three residues Lys-343, Arg-335, and Arg-382, located in DNA binding domain B (DBD-B) of RPA70, were significantly shielded in the hyperphosphorylated protein. Tryptophan fluorescence studies indicated significant quenching of Trp-361, located in the DBD-B domain, induced by hyperphosphorylation of RPA. Consistently, DBD-B became more resistant to the limited proteolysis by chymotrypsin after RPA hyperphosphorylation. Taken together, our results indicate that upon hyperphosphorylation of RPA32 N terminus (RPA32N), RPA undergoes a conformational change involving the single-stranded DNA binding cleft of DBD-B. Comparison of the interactions of native and hyperphosphorylated RPAs with short single-stranded oligonucleotides or partial DNA duplexes with a short 5' or 3' single-stranded DNA tails showed reduced affinity for the latter protein. We propose that the hyperphosphorylation may play a role in modulating the cellular pathways by altering the DBD-B-mediated RPA-DNA and RPA-protein interactions, hypothetically via the interaction of hyperphosphorylated RPA32N with DBD-B.

Denaturation of replication protein A reveals an alternative conformation with intact domain structure and oligonucleotide binding activity

Replication protein A (RPA) is a heterotrimeric, multidomain, single-stranded DNA-binding protein. Using spectroscopic methods and methylene carbene-based chemical modification methods, we have identified conformational intermediates in the denaturation pathway of RPA. Intrinsic protein fluorescence studies reveal unfolding profiles composed of multiple transitions, with midpoints at 1.5, 2.7, 4.2, and 5.3 M urea. CD profiles of RPA unfolding are characterized by a single transition. RPA is stabilized with respect to the CD-monitored transition when bound to a dA15 oligonucleotide. However, oligonucleotide binding appears to exert little, if any, effect on the first fluorescence transition. Methylene carbene chemical modification, coupled with MALDI-TOF mass spectrometry analysis, was also used to monitor unfolding of several specific RPA folds of the protein. The unfolding profiles of the individual structures are characterized by single transitions similar to the CD-monitored transition. Each fold, however, unravels with different individual characteristics, suggesting significant autonomy. Based on results from chemical modification and spectroscopic analyses, we conclude the initial transition observed in fluorescence experiments represents a change in the juxtaposition of binding folds with little unraveling of the domain structures. The second transition represents the unfolding of the majority of fold structure, and the third transition observed by fluorescence correlates with the dissociation of the 70- and 32-kD subunits.

From RPA to BRCA2: lessons from single-stranded DNA binding by the OB-fold

Recent years have witnessed tremendous progress in our structural and biophysical understanding of how replication protein A (RPA), a major nuclear ssDNA-binding protein (SSB), binds DNA. The four ssDNA-binding domains of RPA have the characteristic OB (oligonucleotide/oligosaccharide-binding) fold and contact DNA with specific polarity via a hierarchy-driven dynamic pathway. A growing mass of data suggest that many aspects of the ssDNA binding mechanism are conserved among SSBs of different origin. However, this conservation is not restricted to the SSB class. The concepts of ssDNA binding by the OB-fold, first derived from the RPA structure, have been successfully applied to the functional characterization of the BRCA2 (breast cancer susceptibility gene 2) protein. The BRCA2 structure, in its turn, has helped to better understand RPA function.

Human replication protein A (RPA) binds a primer-template junction in the absence of its major ssDNA-binding domains

The human nuclear single-stranded (ss) DNA- binding protein, replication protein A (RPA), is a heterotrimer consisting of three subunits: p70, p32 and p14. The protein-DNA interaction is mediated by several DNA-binding domains (DBDs): two major (A and B, also known as p70A and p70B) and several minor (C and D, also known as p70C and p32D, and, presumably, by p70N). Here, using crosslinking experiments, we investigated an interaction of RPA deletion mutants containing a subset of the DBDs with partial DNA duplexes containing 5'-protruding ssDNA tails of 10, 20 and 30 nt. The crosslinks were generated using either a 'zero-length' photoreactive group (4-thio-2'-deoxyuridine-5'-monophosphate) embedded in the 3' end of the DNA primer, or a group connected to the 3' end by a lengthy linker (5-[N-[N-(4-azido-2,5-difluoro-3- chloropyridine-6-yl)-3-aminopropionyl]-trans-3-aminopropenyl-1]-2'-deoxyuridine-5'-monophosphate). In the absence of two major DBDs, p70A and p70B, the RPA trimerization core (p70C.p32D.p14) was capable of correctly recognizing the primer- template junction and adopting an orientation similar to that in native RPA. Both p70C and p32D contributed to this recognition. However, the domain contribution differed depending on the size of the ssDNA. In contrast with the trimerization core, the RPA dimerization core (p32D.p14) was incapable of detectably recognizing the DNA- junction structures, suggesting an orchestrating role for p70C in this process.

Independent and coordinated functions of replication protein A tandem high affinity single-stranded DNA binding domains

The initial high affinity binding of single-stranded DNA (ssDNA) by replication protein A (RPA) is involved in the tandem domains in the central region of the RPA70 subunit (RPA70AB). However, it was not clear whether the two domains, RPA70A and RPA70B, bind DNA simultaneously or sequentially. Here, using primarily heteronuclear NMR complemented by fluorescence spectroscopy, we have analyzed the binding characteristics of the individual RPA70A and RPA70B domains and compared them with the intact RPA70AB. NMR chemical shift comparisons confirmed that RPA70A and RPA70B tumble independently in solution in the absence of ssDNA. NMR chemical shift perturbations showed that all ssDNA oligomers bind to the same sites as observed in the x-ray crystal structure of RPA70AB complexed to d(C)8. Titrations using a variety of 5'-mer ssDNA oligomers showed that RPA70A has a 5-10-fold higher affinity for ssDNA than RPA70B. Detailed analysis of ssDNA binding to RPA70A revealed that all DNA sequences interact in a similar mode. Fluorescence binding measurements with a variety of 8-10'-mer DNA sequences showed that RPA70AB interacts with DNA with approximately 100-fold higher affinity than the isolated domains. Calculation of the theoretical "linkage effect" from the structure of RPA70AB suggests that the high overall affinity for ssDNA is a byproduct of the covalent attachment of the two domains via a short flexible tether, which increases the effective local concentration. Taken together, our data are consistent with a sequential model of DNA binding by RPA according to which RPA70A binds the majority of DNA first and subsequent loading of RPA70B domain is facilitated by the linkage effect.

Human replication protein A. The C-terminal RPA70 and the central RPA32 domains are involved in the interactions with the 3'-end of a primer-template DNA

Although the mechanical aspects of the single-stranded DNA (ssDNA) binding activity of human replication protein A (RPA) have been extensively studied, only limited information is available about its interaction with other physiologically relevant DNA structures. RPA interacts with partial DNA duplexes that resemble DNA intermediates found in the processes of DNA replication and DNA repair. Limited proteolysis of RPA showed that RPA associated with ssDNA is less protected against proteases than RPA bound to a partial duplex DNA containing a 5'-protruding tail that had the same length as the ssDNA. Modification of both the 70- and 32-kDa subunits, RPA70 and RPA32, respectively, by photoaffinity labeling indicates that RPA can bind the primer-template junction of partial duplex DNAs by interacting with the 3'-end of the primer. The identification of the protein domains modified by the photoreactive 3'-end of the primer showed that domains located in the central part of the RPA32 subunit (amino acids 39-180) and the C-terminal part of the RPA70 subunit (amino acids 432-616) are involved in these interactions.

Structure of the RPA trimerization core and its role in the multistep DNA-binding mechanism of RPA

The human single-stranded DNA-binding protein, replication protein A (RPA) binds DNA in at least two different modes: initial [8-10 nucleotides (nt)] and stable ( approximately 30 nt). Switching from 8 to 30 nt mode is associated with a large conformational change. Here we report the 2.8 A structure of the RPA trimerization core comprising the C-terminal DNA-binding domain of subunit RPA70 (DBD-C), the central DNA-binding domain of subunit RPA32 (DBD-D) and the entire RPA14 subunit. All three domains are built around a central oligonucleotide/oligosaccharide binding (OB)-fold and flanked by a helix at the C-terminus. Trimerization is mediated by three C-terminal helices arranged in parallel. The OB-fold of DBD-C possesses unique structural features; embedded zinc ribbon and helix-turn-helix motifs. Using time-resolved proteolysis with trypsin, we demonstrate that the trimerization core does not contribute to the binding with substrates of 10 nt, but interacts with oligonucleotides of 24 nt. Taken together, our data indicate that switching from 8-10 to 30 nt mode is mediated by DNA binding with the trimerization core.

Functional analysis of the four DNA binding domains of replication protein A. The role of RPA2 in ssDNA binding

Replication Protein A (RPA), the heterotrimeric single-stranded DNA (ssDNA)-binding protein of eukaryotes, contains four ssDNA binding domains (DBDs) within its two largest subunits, RPA1 and RPA2. We analyzed the contribution of the four DBDs to ssDNA binding affinity by assaying recombinant yeast RPA in which a single DBD (A, B, C, or D) was inactive. Inactivation was accomplished by mutating the two conserved aromatic stacking residues present in each DBD. Mutation of domain A had the most severe effect and eliminated binding to a short substrate such as (dT)12. RPA containing mutations in DBDs B and C bound to substrates (dT)12, 17, and 23 but with reduced affinity compared with wild type RPA. Mutation of DBD-D had little or no effect on the binding of RPA to these substrates. However, mutations in domain D did affect the binding to oligonucleotides larger than 23 nucleotides (nt). Protein-DNA cross-linking indicated that DBD-A (in RPA1) is essential for RPA1 to interact efficiently with substrates of 12 nt or less and that DBD-D (RPA2) interacts efficiently with oligonucleotides of 27 nt or larger. The data support a sequential model of binding in which DBD-A is responsible for the initial interaction with ssDNA, that domains A, B, and C (RPA1) contact 12-23 nt of ssDNA, and that DBD-D (RPA2) is needed for RPA to interact with substrates that are 23-27 nt in length.

Localization of the large subunit of replication factor C near the 5' end of DNA primers

Replication factor C (RFC) is a heteropentameric sliding clamp loader protein essential for processive synthesis of DNA by eukaryotic DNA polymerases delta and epsilon. To study the interaction of RFC with 3' and 5' ends of the DNA primer, we have developed chemical photocrosslinking assay using a synthetic DNA gap and DNA primer-template structures. We have found that the radioactively labeled primers containing a photoreactive group at their 5' end could crosslink with the largest RFC subunit (RFC140) on primer-templates and DNA gap structures, but that 3' end photoreactive primers could only crosslink with RFC140 within the DNA gap structure. Addition of replication protein A (RPA) to the reaction mixture resulted in the crosslinking of RPA subunits and inhibited crosslinking of RFC140 using 3' but not 5' photoreactive primers present at the gap. The results suggest specific contacts between RFC140 and the 5' end of the DNA primer. Together with previous data, these experiments allow us to propose a model for the DNA polymerase switch during eukaryotic DNA replication.

Replication protein A as a "fidelity clamp" for DNA polymerase alpha

The current view of DNA replication in eukaryotes predicts that DNA polymerase alpha (pol alpha)-primase synthesizes the first 10-ribonucleotide-long RNA primer on the leading strand and at the beginning of each Okazaki fragment on the lagging strand. Subsequently, pol alpha elongates such an RNA primer by incorporating about 20 deoxynucleotides. pol alpha displays a low processivity and, because of the lack of an intrinsic or associated 3'--> 5' exonuclease activity, it is more error-prone than other replicative pols. Synthesis of the RNA/DNA primer catalyzed by pol alpha-primase is a critical step in the initiation of DNA synthesis, but little is known about the role of the DNA replication accessory proteins in its regulation. In this paper we provide evidences that the single-stranded DNA-binding protein, replication protein A (RP-A), acts as an auxiliary factor for pol alpha playing a dual role: (i) it stabilizes the pol alpha/primer complex, thus acting as a pol clamp; and (ii) it significantly reduces the misincorporation efficiency by pol alpha. Based on these results, we propose a hypothetical model in which RP-A is involved in the regulation of the early events of DNA synthesis by acting as a "fidelity clamp" for pol alpha.

Structure of the major single-stranded DNA-binding domain of replication protein A suggests a dynamic mechanism for DNA binding

Although structures of single-stranded (ss)DNA-binding proteins (SSBs) have been reported with and without ssDNA, the mechanism of ssDNA binding in eukarya remains speculative. Here we report a 2.5 Angstroms structure of the ssDNA-binding domain of human replication protein A (RPA) (eukaryotic SSB), for which we previously reported a structure in complex with ssDNA. A comparison of free and bound forms of RPA revealed that ssDNA binding is associated with a major reorientation between, and significant conformational changes within, the structural modules--OB-folds--which comprise the DNA-binding domain. Two OB-folds, whose tandem orientation was stabilized by the presence of DNA, adopted multiple orientations in its absence. Within the OB-folds, extended loops implicated in DNA binding significantly changed conformation in the absence of DNA. Analysis of intermolecular contacts suggested the possibility that other RPA molecules and/or other proteins could compete with DNA for the same binding site. Using this mechanism, protein-protein interactions can regulate, and/or be regulated by DNA binding. Combined with available biochemical data, this structure also suggested a dynamic model for the DNA-binding mechanism.

Polarity of human replication protein A binding to DNA

Replication protein A (RPA), the nuclear single-stranded DNA binding protein is involved in DNA replication, nucleotide excision repair (NER) and homologous recombination. It is a stable heterotrimer consisting of subunits with molecular masses of 70, 32 and 14 kDa (p70, p32 and p14, respectively). Gapped DNA structures are common intermediates during DNA replication and NER. To analyze the interaction of RPA and its subunits with gapped DNA we designed structures containing 9 and 30 nucleotide gaps with a photoreactive arylazido group at the 3'-end of the upstream oligonucleotide or at the 5'-end of the downstream oligonucleotide. UV crosslinking and subsequent analysis showed that the p70 subunit mainly interacts with the 5'-end of DNA irrespective of DNA structure, while the subunit orientation towards the 3'-end of DNA in the gap structures strongly depends on the gap size. The results are compared with the data obtained previously with the primer-template systems containing 5'- or 3'-protruding DNA strands. Our results suggest a model of polar RPA binding to the gapped DNA.