Backbone and side-chain ¹H, ¹³C and ¹⁵N resonance assignments of the OB domain of the single stranded DNA binding protein from Sulfolobus solfataricus and chemical shift mapping of the DNA-binding interface

NMR Structure and Biophysical Characterization of Thermophilic Single-Stranded DNA Binding Protein from Sulfolobus Solfataricus

Proteins from Sulfolobus solfataricus (S. solfataricus), an extremophile, are active even at high temperatures. The single-stranded DNA (ssDNA) binding protein of S. solfataricus (SsoSSB) is overexpressed to protect ssDNA during DNA metabolism. Although SsoSSB has the potential to be applied in various areas, its structural and ssDNA binding properties at high temperatures have not been studied. We present the solution structure, backbone dynamics, and ssDNA binding properties of SsoSSB at 50 °C. The overall structure is consistent with the structures previously studied at room temperature. However, the loop between the first two β sheets, which is flexible and is expected to undergo conformational change upon ssDNA binding, shows a difference from the ssDNA bound structure. The ssDNA binding ability was maintained at high temperature, but different interactions were observed depending on the temperature. Backbone dynamics at high temperature showed that the rigidity of the structured region was well maintained. The investigation of an N-terminal deletion mutant revealed that it is important for maintaining thermostability, structure, and ssDNA binding ability. The structural and dynamic properties of SsoSSB observed at high temperature can provide information on the behavior of proteins in thermophiles at the molecular level and guide the development of new experimental techniques.

Resonance assignments and secondary structure of thermophile single-stranded DNA binding protein from Sulfolobus solfataricus at 323K

Single-stranded DNA (ssDNA)-binding proteins (SSBs) are essential for DNA replication, recombination, and repair processes in all organisms. Sulfolobus solfataricus (S. solfataricus), a hyperthermophilic species, overexpresses its SSB (S. solfataricus SSB (SsoSSB)) to protect ssDNA during DNA metabolisms. Even though the crystal structure of apo SsoSSB and its ssDNA-bound solution structure have been reported at room temperature, structural information at high temperature is not yet available. To find out how SsoSSB maintains its structure and ssDNA binding affinity at high temperatures, we performed multidimensional NMR experiments for SsoSSB at 323K. In this study, we present the backbone and side chain chemical shifts and predict the secondary structure of SsoSSB from the chemical shifts. We found that SsoSSB is ordered, even at high temperatures, and has the same fold at high temperature as at room temperature. Our data will help improve structural analyses and our understanding of the features of thermophilic proteins.

Single-Stranded DNA-Binding Proteins in the Archaea

Single-stranded (ss) DNA-binding proteins are found in all three domains of life where they play vital roles in nearly all aspects of DNA metabolism by binding to and stabilizing exposed ssDNA and acting as platforms onto which DNA-processing activities can assemble. The ssDNA-binding factors SSB and RPA are extremely well conserved across bacteria and eukaryotes, respectively, and comprise one or more OB-fold ssDNA-binding domains. In the third domain of life, the archaea, multiple types of ssDNA-binding protein are found with a variety of domain architectures and subunit compositions, with OB-fold ssDNA-binding domains being a characteristic of most, but not all. This chapter summarizes current knowledge of the distribution, structure, and biological function of the archaeal ssDNA-binding factors, highlighting key features shared between clades and those that distinguish the proteins of different clades from one another. The likely cellular functions of the proteins are discussed and gaps in current knowledge identified.

Backbone 1H, 13C and 15N resonance assignments of the OB domain of the single stranded DNA-binding protein hSSB2 (NABP1/OBFC2A) and chemical shift mapping of the DNA-binding interface

Single stranded DNA-binding proteins (SSBs) are essential for the maintenance of genome integrity and are required in in all known cellular organisms. Over the last 10 years, the role of two new human SSBs, hSSB1 (NABP2/OBFC2B) and hSSB2 (NABP1/OBFC2A), has been described and characterised in various important DNA repair processes. Both these proteins are made up of a conserved oligonucleotide-binding (OB) fold that is responsible for ssDNA recognition as well a unique flexible carboxy-terminal extension involved in protein-protein interactions. Due to their similar domain organisation, hSSB1 and hSSB2 have been found to display some overlapping functions. However, several studies have also revealed cell- and tissue-specific roles for these two proteins, most likely due to small but significant differences in the protein sequence of the OB domains. While the molecular details of ssDNA binding by hSSB1 has been studied extensively, comparatively little is known about hSSB2. In this study, we use NMR solution-state backbone resonance assignments of the OB domain of hSSB2 to map the ssDNA interaction interface. Our data reveal that ssDNA binding by hSSB2 is driven by four key aromatic residues in analogy to hSSB1, however, some significant differences in the chemical shift perturbations are observed, reflecting differences in ssDNA recognition. Future studies will aim at determining the structural basis of these differences and thus help to gain a more comprehensive understanding of the functional divergences that these novel hSSBs display in the context of genome maintenance.

Staphylococcus aureus single-stranded DNA-binding protein SsbA can bind but cannot stimulate PriA helicase

Single-stranded DNA-binding protein (SSB) and PriA helicase play important roles in bacterial DNA replication restart process. The mechanism by which PriA helicase is bound and stimulated by SSB in Escherichia coli (Ec) has been established, but information on this process in Gram-positive bacteria are limited. We characterized the properties of SSB from Staphylococcus aureus (SaSsbA, a counterpart of EcSSB) and analyzed its interaction with SaPriA. The gel filtration chromatography analysis of purified SaSsbA showed a stable tetramer in solution. The crystal structure of SaSsbA determined at 1.82 Å resolution (PDB entry 5XGT) reveals that the classic oligonucleotide/oligosaccharide-binding folds are formed in the N-terminal DNA-binding domain, but the entire C-terminal domain is disordered. Unlike EcSSB, which can stimulate EcPriA via a physical interaction between EcPriA and the C-terminus of EcSSB (SSB-Ct), SaSsbA does not affect the activity of SaPriA. We also found that SaPriA can be bound by SaSsbA, but not by SaSsbA-Ct. Although no effect was found with SaSsbA, SaPriA can be significantly stimulated by the Gram-negative Klebsiella pneumoniae SSB (KpSSB). In addition, we found that the conserved SSB-Ct binding site of KpPriA (Trp82, Tyr86, Lys370, Arg697, and Gln701) is not present in SaPriA. Arg697 in KpPriA is known to play a critical role in altering the SSB₃₅/SSB₆₅ distribution, but this corresponding residue in SaPriA is Glu767 instead, which has an opposite charge to Arg. SaPriA E767R mutant was constructed and analyzed; however, it still cannot be stimulated by SaSsbA. Finally, we found that the conserved MDFDDDIPF motif in the Gram-negative bacterial SSB is DISDDDLPF in SaSsbA, i.e., F172 in EcSSB and F168 in KpSSB is S161 in SaSsbA, not F. When acting with SaSsbA S161F mutant, the activity of SaPriA was dramatically enhanced elevenfold. Overall, the conserved binding sites, both in EcPriA and EcSSB, are not present in SaPriA and SaSsbA, thereby no stimulation occurs. Our observations through structure-sequence comparison and mutational analyses indicate that the case of EcPriA-EcSSB is not applicable to SaPriA-SaSsbA because of inherent differences among the species.

Backbone (1)H, (13)C and (15)N resonance assignments of the OB domain of the single stranded DNA-binding protein hSSB1 (NABP2/OBFC2B) and chemical shift mapping of the DNA-binding interface

Single-stranded DNA-binding proteins (SSBs) are highly important in DNA metabolism and play an essential role in all major DNA repair pathways. SSBs are generally characterised by the presence of an oligonucleotide binding (OB) fold which is able to recognise single-stranded DNA (ssDNA) with high affinity. We discovered two news SSBs in humans (hSSB1 and hSSB2) that both contain a single OB domain followed by a divergent spacer region and a charged C-terminus. We have extensively characterised one of these, hSSB1 (NABP2/OBFC2B), in numerous important DNA processing events such as, in DNA double-stranded break repair and in the response to oxidative DNA damage. Although the structure of hSSB1 bound to ssDNA has recently been determined using X-ray crystallography, the detailed atomic level mechanism of the interaction of hSSB1 with ssDNA in solution has not been established. In this study we report the solution-state backbone chemical shift assignments of the OB domain of hSSB1. In addition, we have utilized NMR to map the DNA-binding interface of hSSB1, revealing major differences between recognition of ssDNA under physiological conditions and in the recently determined crystal structure. Our NMR data in combination with further biophysical and biochemical experiments will allow us to address these discrepancies and shed light onto the structural basis of DNA-binding by hSSB1 in solution.

The structural basis of DNA binding by the single-stranded DNA-binding protein from Sulfolobus solfataricus

We present the 3D solution structure of the canonical SSB from the crenarchaeote Sulfolobus solfataricus bound to single-stranded DNA and compare this structure with human homologues.

C-terminal domain swapping of SSB changes the size of the ssDNA binding site

Single-stranded DNA-binding protein (SSB) plays an important role in DNA metabolism, including DNA replication, repair, and recombination, and is therefore essential for cell survival. Bacterial SSB consists of an N-terminal ssDNA-binding/oligomerization domain and a flexible C-terminal protein-protein interaction domain. We characterized the ssDNA-binding properties of Klebsiella pneumoniae SSB (KpSSB), Salmonella enterica Serovar Typhimurium LT2 SSB (StSSB), Pseudomonas aeruginosa PAO1 SSB (PaSSB), and two chimeric KpSSB proteins, namely, KpSSBnStSSBc and KpSSBnPaSSBc. The C-terminal domain of StSSB or PaSSB was exchanged with that of KpSSB through protein chimeragenesis. By using the electrophoretic mobility shift assay, we characterized the stoichiometry of KpSSB, StSSB, PaSSB, KpSSBnStSSBc, and KpSSBnPaSSBc, complexed with a series of ssDNA homopolymers. The binding site sizes were determined to be 26 ± 2, 21 ± 2, 29 ± 2, 21 ± 2, and 29 ± 2 nucleotides (nt), respectively. Comparison of the binding site sizes of KpSSB, KpSSBnStSSBc, and KpSSBnPaSSBc showed that the C-terminal domain swapping of SSB changes the size of the binding site. Our observations suggest that not only the conserved N-terminal domain but also the C-terminal domain of SSB is an important determinant for ssDNA binding.