Transient association between proteins elicits alteration of dynamics at sites far away from interfaces.

Proteins are known to undergo structural changes upon binding to partner proteins. However, the prevalence, extent, location, and function of change in protein dynamics due to transient protein-protein interactions is not well documented. Here, we have analyzed a dataset of 58 protein-protein complexes of known three-dimensional structure and structures of their corresponding unbound forms to evaluate dynamics changes induced by binding. Fifty-five percent of cases showed significant dynamics change away from the interfaces. This change is not always accompanied by an observed structural change. Binding of protein partner is found to alter inter-residue communication within the tertiary structure in about 90% of cases. Also, residue motions accessible to proteins in unbound form were not always maintained in the bound form. Further analyses revealed functional roles for the distant site where dynamics change was observed. Overall, the results presented here strongly suggest that alteration of protein dynamics due to binding of a partner protein commonly occurs.

Structural insights into assembly, operation and inhibition of a type I restriction-modification system.

Type I restriction-modification (R-M) systems are widespread in prokaryotic genomes and provide robust protection against foreign DNA. They are multisubunit enzymes with methyltransferase, endonuclease and translocase activities. Despite extensive studies over the past five decades, little is known about the molecular mechanisms of these sophisticated machines. Here, we report the cryo-electron microscopy structures of the representative EcoR124I R-M system in different assemblies (R2M2S1, R1M2S1 and M2S1) bound to target DNA and the phage and mobile genetic element-encoded anti-restriction proteins Ocr and ArdA. EcoR124I can precisely regulate different enzymatic activities by adopting distinct conformations. The marked conformational transitions of EcoR124I are dependent on the intrinsic flexibility at both the individual-subunit and assembled-complex levels. Moreover, Ocr and ArdA use a DNA-mimicry strategy to inhibit multiple activities, but do not block the conformational transitions of the complexes. These structural findings, complemented by mutational studies of key intermolecular contacts, provide insights into assembly, operation and inhibition mechanisms of type I R-M systems.

Mining the Methylome Reveals Extensive Diversity in Staphylococcus epidermidis Restriction Modification.

Staphylococcus epidermidis is a significant opportunistic pathogen of humans. Molecular studies in this species have been hampered by the presence of restriction-modification (RM) systems that limit introduction of foreign DNA. Here, we establish the complete genomes and methylomes for seven clinically significant, genetically diverse S. epidermidis isolates and perform the first systematic genomic analyses of the type I RM systems within both S. epidermidis and Staphylococcus aureus Our analyses revealed marked differences in the gene arrangement, chromosomal location, and movement of type I RM systems between the two species. Unlike S. aureus, S. epidermidis type I RM systems demonstrate extensive diversity even within a single genetic lineage. This is contrary to current assumptions and has important implications for approaching the genetic manipulation of S. epidermidis Using Escherichia coli plasmid artificial modification (PAM) to express S. epidermidishsdMS, we readily overcame restriction barriers in S. epidermidis and achieved electroporation efficiencies equivalent to those of modification-deficient mutants. With these functional experiments, we demonstrated how genomic data can be used to predict both the functionality of type I RM systems and the potential for a strain to be electroporation proficient. We outline an efficient approach for the genetic manipulation of S. epidermidis strains from diverse genetic backgrounds, including those that have hitherto been intractable. Additionally, we identified S. epidermidis BPH0736, a naturally restriction-defective, clinically significant, multidrug-resistant ST2 isolate, as an ideal candidate for molecular studies.IMPORTANCEStaphylococcus epidermidis is a major cause of hospital-acquired infections, especially those related to implanted medical devices. Understanding how S. epidermidis causes disease and devising ways to combat these infections have been hindered by an inability to genetically manipulate clinically significant hospital-adapted strains. Here, we provide the first comprehensive analyses of the barriers to the uptake of foreign DNA in S. epidermidis and demonstrate that these are distinct from those described for S. aureus Using these insights, we demonstrate an efficient approach for the genetic manipulation of S. epidermidis to enable the study of clinical isolates for the first time.

A model for the evolution of prokaryotic DNA restriction-modification systems based upon the structural malleability of Type I restriction-modification enzymes.

Restriction Modification (RM) systems prevent the invasion of foreign genetic material into bacterial cells by restriction and protect the host's genetic material by methylation. They are therefore important in maintaining the integrity of the host genome. RM systems are currently classified into four types (I to IV) on the basis of differences in composition, target recognition, cofactors and the manner in which they cleave DNA. Comparing the structures of the different types, similarities can be observed suggesting an evolutionary link between these different types. This work describes the 'deconstruction' of a large Type I RM enzyme into forms structurally similar to smaller Type II RM enzymes in an effort to elucidate the pathway taken by Nature to form these different RM enzymes. Based upon the ability to engineer new enzymes from the Type I 'scaffold', an evolutionary pathway and the evolutionary pressures required to move along the pathway from Type I RM systems to Type II RM systems are proposed. Experiments to test the evolutionary model are discussed.

Crystal structure of a novel domain of the motor subunit of the Type I restriction enzyme EcoR124 involved in complex assembly and DNA binding.

Although EcoR124 is one of the better-studied Type I restriction-modification enzymes, it still presents many challenges to detailed analyses because of its structural and functional complexity and missing structural information. In all available structures of its motor subunit HsdR, responsible for DNA translocation and cleavage, a large part of the HsdR C terminus remains unresolved. The crystal structure of the C terminus of HsdR, obtained with a crystallization chaperone in the form of pHluorin fusion and refined to 2.45 Å, revealed that this part of the protein forms an independent domain with its own hydrophobic core and displays a unique α-helical fold. The full-length HsdR model, based on the WT structure and the C-terminal domain determined here, disclosed a proposed DNA-binding groove lined by positively charged residues. In vivo and in vitro assays with a C-terminal deletion mutant of HsdR supported the idea that this domain is involved in complex assembly and DNA binding. Conserved residues identified through sequence analysis of the C-terminal domain may play a key role in protein-protein and protein-DNA interactions. We conclude that the motor subunit of EcoR124 comprises five structural and functional domains, with the fifth, the C-terminal domain, revealing a unique fold characterized by four conserved motifs in the IC subfamily of Type I restriction-modification systems. In summary, the structural and biochemical results reported here support a model in which the C-terminal domain of the motor subunit HsdR of the endonuclease EcoR124 is involved in complex assembly and DNA binding.

A residue of motif III positions the helicase domains of motor subunit HsdR in restriction-modification enzyme EcoR124I.

Type I restriction-modification enzymes differ significantly from the type II enzymes commonly used as molecular biology reagents. On hemi-methylated DNAs type I enzymes like the EcoR124I restriction-modification complex act as conventional adenine methylases at their specific target sequences, but unmethylated targets induce them to translocate thousands of base pairs through the stationary enzyme before cleaving distant sites nonspecifically. EcoR124I is a superfamily 2 DEAD-box helicase like eukaryotic double-strand DNA translocase Rad54, with two RecA-like helicase domains and seven characteristic sequence motifs that are implicated in translocation. In Rad54 a so-called extended region adjacent to motif III is involved in ATPase activity. Although the EcoR124I extended region bears sequence and structural similarities with Rad54, it does not influence ATPase or restriction activity as shown in this work, but mutagenesis of the conserved glycine residue of its motif III does alter ATPase and DNA cleavage activity. Through the lens of molecular dynamics, a full model of HsdR of EcoR124I based on available crystal structures allowed interpretation of functional effects of mutants in motif III and its extended region. The results indicate that the conserved glycine residue of motif III has a role in positioning the two helicase domains.

Structure and dynamics of mesophilic variants from the homing endonuclease I-DmoI.

I-DmoI, from the hyperthermophilic archaeon Desulfurococcus mobilis, belongs to the LAGLIDADG homing endonuclease protein family. Its members are highly specific enzymes capable of recognizing long DNA target sequences, thus providing potential tools for genome manipulation. Working towards this particular application, many efforts have been made to generate mesophilic variants of I-DmoI that function at lower temperatures than the wild-type. Here, we report a structural and computational analysis of two I-DmoI mesophilic mutants. Despite very limited structural variations between the crystal structures of these variants and the wild-type, a different dynamical behaviour near the cleavage sites is observed. In particular, both the dynamics of the water molecules and the protein perturbation effect on the cleavage site correlate well with the changes observed in the experimental enzymatic activity.

[The importance of C-terminal aspartic acid residue (D141) to the antirestriction activity of the ArdB (R64) protein].

Antirestriction proteins of the ArdB/KlcA family are specific inhibitors of restriction (endonuclease) activity of type-I restriction/modification enzymes. The effect of conserved amino acid residues on the antirestriction activity of the ArdB protein encoded by the transmissible R64 (IncI1) plasmid has been investigated. An analysis of the amino acid sequences of ArdB homologues demonstrated the presence of four groups of conserved residues ((1) R16, E32, and W51; (2) Y46 and G48; (3) S81, D83 and E132, and (4) N77, L(I)140, and D141) on the surface of the protein globule. Amino acid residues of the fourth group showed a unique localization pattern with the terminal residue protruding beyond the globule surface. The replacement of two conserved amino acids (D141 and N77) located in the close vicinity of each other on the globule surface showed that the C-terminal D141 is essential for the antirestriction activity of ArdB. The deletion of this residue, as well as replacement by a hydrophobic threonine residue (D141T), completely abolished the antirestriction activity of ArdB. The synonymous replacement of D141 by a glutamic acid residue (D141E) caused an approximately 30-fold decrease of the antirestriction activity of ArdB, and the point mutation N77A caused an approximately 20-fold decrease in activity. The residues D141 and N77 located on the surface of the protein globule are presumably essential for the formation of a contact between ArdB and a currently unknown factor that modulates the activity of type-I restriction/modification enzymes.

DNA target recognition domains in the Type I restriction and modification systems of Staphylococcus aureus.

Staphylococcus aureus displays a clonal population structure in which horizontal gene transfer between different lineages is extremely rare. This is due, in part, to the presence of a Type I DNA restriction-modification (RM) system given the generic name of Sau1, which maintains different patterns of methylation on specific target sequences on the genomes of different lineages. We have determined the target sequences recognized by the Sau1 Type I RM systems present in a wide range of the most prevalent S. aureus lineages and assigned the sequences recognized to particular target recognition domains within the RM enzymes. We used a range of biochemical assays on purified enzymes and single molecule real-time sequencing on genomic DNA to determine these target sequences and their patterns of methylation. Knowledge of the main target sequences for Sau1 will facilitate the synthesis of new vectors for transformation of the most prevalent lineages of this 'untransformable' bacterium.

pHluorin-assisted expression, purification, crystallization and X-ray diffraction data analysis of the C-terminal domain of the HsdR subunit of the Escherichia coli type I restriction-modification system EcoR124I.

The HsdR subunit of the type I restriction-modification system EcoR124I is responsible for the translocation as well as the restriction activity of the whole complex consisting of the HsdR, HsdM and HsdS subunits, and while crystal structures are available for the wild type and several mutants, the C-terminal domain comprising approximately 150 residues was not resolved in any of these structures. Here, three fusion constructs with the GFP variant pHluorin developed to overexpress, purify and crystallize the C-terminal domain of HsdR are reported. The shortest of the three encompassed HsdR residues 887-1038 and yielded crystals that belonged to the orthorhombic space group C2221, with unit-cell parameters a = 83.42, b = 176.58, c = 126.03 Å, α = ß = γ = 90.00° and two molecules in the asymmetric unit (VM = 2.55 Å(3) Da(-1), solvent content 50.47%). X-ray diffraction data were collected to a resolution of 2.45 Å.

Effect of DNA on the conformational dynamics of the endonucleases I-DmoI as provided by molecular dynamics simulations.

The conformational behavior of the wild-type endonucleases I-DmoI and two of its mutants has been studied in the presence and in the absence of DNA target sequences by means of extended molecular dynamics simulations. Our results show that in the absence of DNA, the three protein forms explore a similar essential conformational space, whereas when bound to the same DNA target sequence of 25 base pairs, they diversify and restrain the subspace explored. In addition, the differences in the essential subspaces explored by the residues near the catalytic site for both the bound and unbound forms are discussed in background of the experimental protein activity.

Expression and purification of the modification-dependent restriction enzyme BisI and its homologous enzymes.

The methylation-dependent restriction endonuclease (REase) BisI (G(m5)C ↓ NGC) is found in Bacillus subtilis T30. We expressed and purified the BisI endonuclease and 34 BisI homologs identified in bacterial genomes. 23 of these BisI homologs are active based on digestion of (m5)C-modified substrates. Two major specificities were found among these BisI family enzymes: Group I enzymes cut GCNGC containing two to four (m5)C in the two strands, or hemi-methylated sites containing two (m5)C in one strand; Group II enzymes only cut GCNGC sites containing three to four (m5)C, while one enzyme requires all four cytosines to be modified for cleavage. Another homolog, Esp638I cleaves GCS ↓ SGC (relaxed specificity RCN ↓ NGY, containing at least four (m5)C). Two BisI homologs show degenerate specificity cleaving unmodified DNA. Many homologs are small proteins ranging from 150 to 190 amino acid (aa) residues, but some homologs associated with mobile genetic elements are larger and contain an extra C-terminal domain. More than 156 BisI homologs are found in >60 bacterial genera, indicating that these enzymes are widespread in bacteria. They may play an important biological function in restricting pre-modified phage DNA.

Key Players in I-DmoI Endonuclease Catalysis Revealed from Structure and Dynamics.

Homing endonucleases, such as I-DmoI, specifically recognize and cleave long DNA target sequences (∼20 bp) and are potentially powerful tools for genome manipulation. However, inefficient and off-target DNA cleavage seriously limits specific editing in complex genomes. One approach to overcome these limitations is to unambiguously identify the key structural players involved in catalysis. Here, we report the E117A I-DmoI mutant crystal structure at 2.2 Šresolution that, together with the wt and Q42A/K120M constructs, is combined with computational approaches to shed light on protein cleavage activity. The cleavage mechanism was related both to key structural effects, such as the position of water molecules and ions participating in the cleavage reaction, and to dynamical effects related to protein behavior. In particular, we found that the protein perturbation pattern significantly changes between cleaved and noncleaved DNA strands when the ions and water molecules are correctly positioned for the nucleophilic attack that initiates the cleavage reaction, in line with experimental enzymatic activity. The proposed approach paves the way for an effective, general, and reliable procedure to analyze the enzymatic activity of endonucleases from a very limited data set, i.e., structure and dynamics.

De novo design of protein mimics of B-DNA.

Structural mimicry of DNA is utilized in nature as a strategy to evade molecular defences mounted by host organisms. One such example is the protein Ocr - the first translation product to be expressed as the bacteriophage T7 infects E. coli. The structure of Ocr reveals an intricate and deliberate arrangement of negative charges that endows it with the ability to mimic ∼24 base pair stretches of B-DNA. This uncanny resemblance to DNA enables Ocr to compete in binding the type I restriction modification (R/M) system, and neutralizes the threat of hydrolytic cleavage of viral genomic material. Here, we report the de novo design and biophysical characterization of DNA mimicking peptides, and describe the inhibitory action of the designed helical bundles on a type I R/M enzyme, EcoR124I. This work validates the use of charge patterning as a design principle for creation of protein mimics of DNA, and serves as a starting point for development of therapeutic peptide inhibitors against human pathogens that employ molecular camouflage as part of their invasion stratagem.

Engineering a Nickase on the Homing Endonuclease I-DmoI Scaffold.

Homing endonucleases are useful tools for genome modification because of their capability to recognize and cleave specifically large DNA targets. These endonucleases generate a DNA double strand break that can be repaired by the DNA damage response machinery. The break can be repaired by homologous recombination, an error-free mechanism, or by non-homologous end joining, a process susceptible to introducing errors in the repaired sequence. The type of DNA cleavage might alter the balance between these two alternatives. The use of "nickases" producing a specific single strand break instead of a double strand break could be an approach to reduce the toxicity associated with non-homologous end joining by promoting the use of homologous recombination to repair the cleavage of a single DNA break. Taking advantage of the sequential DNA cleavage mechanism of I-DmoI LAGLIDADG homing endonuclease, we have developed a new variant that is able to cut preferentially the coding DNA strand, generating a nicked DNA target. Our structural and biochemical analysis shows that by decoupling the action of the catalytic residues acting on each strand we can inhibit one of them while keeping the other functional.

Functional coupling of duplex translocation to DNA cleavage in a type I restriction enzyme.

Type I restriction-modification enzymes are multifunctional heteromeric complexes with DNA cleavage and ATP-dependent DNA translocation activities located on motor subunit HsdR. Functional coupling of DNA cleavage and translocation is a hallmark of the Type I restriction systems that is consistent with their proposed role in horizontal gene transfer. DNA cleavage occurs at nonspecific sites distant from the cognate recognition sequence, apparently triggered by stalled translocation. The X-ray crystal structure of the complete HsdR subunit from E. coli plasmid R124 suggested that the triggering mechanism involves interdomain contacts mediated by ATP. In the present work, in vivo and in vitro activity assays and crystal structures of three mutants of EcoR124I HsdR designed to probe this mechanism are reported. The results indicate that interdomain engagement via ATP is indeed responsible for signal transmission between the endonuclease and helicase domains of the motor subunit. A previously identified sequence motif that is shared by the RecB nucleases and some Type I endonucleases is implicated in signaling.

Vibronic structure in the far-UV electronic circular dichroism spectra of proteins.

The Franck-Condon effect is considered and the vibrational structure of the πnbπ* transition of the peptide backbone is incorporated into matrix method calculations of the electronic circular dichroism (CD) spectra of proteins in the far-ultraviolet. We employ the state-averaged CASPT2 method to calculate the ground and πnbπ* excited state geometries and frequencies of N-methylacetamide (NMA), which represents the peptide chromophore. The results of these calculations are used to incorporate vibronic levels of the excited states into the matrix method calculation. The CD spectra of a set of 49 proteins, comprising a range of structural types, are calculated to assess the influence of the vibrational structure. The calculated spectra of α-helical proteins are better resolved using the vibronic parameters and correlation between the experimental and the calculated intensity of less regular ß structure proteins improves over most wavelengths in the far-UV. No obvious improvement is observed in the calculated spectra of regular ß-sheet proteins. Our high-level ab initio calculations of the vibronic structure of the πnbπ* transition in NMA have provided some further insight into the physical origins of the nature of protein CD spectra in the far-UV.

Interdomain communication in the endonuclease/motor subunit of type I restriction-modification enzyme EcoR124I.

Restriction-modification systems protect bacteria from foreign DNA. Type I restriction-modification enzymes are multifunctional heteromeric complexes with DNA-cleavage and ATP-dependent DNA translocation activities located on endonuclease/motor subunit HsdR. The recent structure of the first intact motor subunit of the type I restriction enzyme from plasmid EcoR124I suggested a mechanism by which stalled translocation triggers DNA cleavage via a lysine residue on the endonuclease domain that contacts ATP bound between the two helicase domains. In the present work, molecular dynamics simulations are used to explore this proposal. Molecular dynamics simulations suggest that the Lys-ATP contact alternates with a contact with a nearby loop housing the conserved QxxxY motif that had been implicated in DNA cleavage. This model is tested here using in vivo and in vitro experiments. The results indicate how local interactions are transduced to domain motions within the endonuclease/motor subunit.

Carbon nanoparticle-protected aptamers for highly sensitive and selective detection of biomolecules based on nuclease-assisted target recycling signal amplification.

Based on the protective properties of carbon nanoparticles for aptamers against the digestion of nuclease, we have developed a nuclease-assisted target recycling signal amplification method for highly sensitive detection of biomolecules, such as ATP and kanamycin. The high binding specificity between aptamers and targets leads to excellent selectivity of the assay.

Cloning, crystallization and preliminary X-ray diffraction analysis of an intact DNA methyltransferase of a type I restriction-modification enzyme from Vibrio vulnificus.

Independently of the restriction (HsdR) subunit, the specificity (HsdS) and methylation (HsdM) subunits interact with each other, and function as a methyltransferase in type I restriction-modification systems. A single gene that combines the HsdS and HsdM subunits in Vibrio vulnificus YJ016 was expressed and purified. A crystal suitable for X-ray diffraction was obtained from 25%(w/v) polyethylene glycol monomethylether 5000, 0.1 M HEPES pH 8.0, 0.2 M ammonium sulfate at 291 K by hanging-drop vapour diffusion. Diffraction data were collected to a resolution of 2.31 Šusing synchrotron radiation. The crystal belonged to the primitive monoclinic space group P21, with unit-cell parameters a = 93.25, b = 133.04, c = 121.49 Å, ß = 109.7°. With four molecules in the asymmetric unit, the crystal volume per unit protein weight was 2.61 Å(3) Da(-1), corresponding to a solvent content of 53%.