Structure based hyperthermostability of archaeal histone HPhA from Pyrococcus horikoshii

A comprehensive review on histone-mediated transfection for gene therapy

Histone has been considered to be an effective carrier in non-viral gene delivery due to its unique properties such as efficient DNA binding ability, direct translocation to cytoplasm and favorable nuclear localization ability. Meanwhile, the rapid development of genetic engineering techniques could facilitate the construction of multifunctional fusion proteins based on histone molecules to further improve the transfection efficiency. Remarkably, histone has been demonstrated to achieve gene transfection in a synergistic manner with cationic polymers, affording to a significant improvement of transfection efficiency. In the review, we highlighted the recent developments and future trends in gene delivery mediated by histones or histone-based fusion proteins/peptides. This review also discussed the mechanism of histone-mediated gene transfection and provided an outlook for future therapeutic opportunities in the viewpoint of transfection efficacy and biosafety.

Developing a Novel Gene-Delivery Vector System Using the Recombinant Fusion Protein of Pseudomonas Exotoxin A and Hyperthermophilic Archaeal Histone HPhA

Non-viral gene delivery system with many advantages has a great potential for the future of gene therapy. One inherent obstacle of such approach is the uptake by endocytosis into vesicular compartments. Receptor-mediated gene delivery method holds promise to overcome this obstacle. In this study, we developed a receptor-mediated gene delivery system based on a combination of the Pseudomonas exotoxin A (PE), which has a receptor binding and membrane translocation domain, and the hyperthermophilic archaeal histone (HPhA), which has the DNA binding ability. First, we constructed and expressed the rPE-HPhA fusion protein. We then examined the cytotoxicity and the DNA binding ability of rPE-HPhA. We further assessed the efficiency of transfection of the pEGF-C1 plasmid DNA to CHO cells by the rPE-HPhA system, in comparison to the cationic liposome method. The results showed that the transfection efficiency of rPE-HPhA was higher than that of cationic liposomes. In addition, the rPE-HPhA gene delivery system is non-specific to DNA sequence, topology or targeted cell type. Thus, the rPE-HPhA system can be used for delivering genes of interest into mammalian cells and has great potential to be applied for gene therapy.

An archaeal histone is required for transformation of Thermococcus kodakarensis

Archaeal histones wrap DNA into complexes, designated archaeal nucleosomes, that resemble the tetrasome core of a eukaryotic nucleosome. Therefore, all DNA interactions in vivo in Thermococcus kodakarensis, the most genetically versatile model species for archaeal research, must occur in the context of a histone-bound genome. Here we report the construction and properties of T. kodakarensis strains that have TK1413 or TK2289 deleted, the genes that encode HTkA and HTkB, respectively, the two archaeal histones present in this archaeon. All attempts to generate a strain with both TK1413 and TK2289 deleted were unsuccessful, arguing that a histone-mediated event(s) in T. kodakarensis is essential. The HTkA and HTkB amino acid sequences are 84% identical (56 of 67 residues) and 94% similar (63 of 67 residues), but despite this homology and their apparent redundancy in terms of supporting viability, the absence of HTkA and HTkB resulted in differences in growth and in quantitative and qualitative differences in genome transcription. A most surprising result was that the deletion of TK1413 (ΔhtkA) resulted in a T. kodakarensis strain that was no longer amenable to transformation, whereas the deletion of TK2289 (ΔhtkB) had no detrimental effects on transformation. Potential roles for the archaeal histones in regulating gene expression and for HTkA in DNA uptake and recombination are discussed.

Nanoarchaeal origin of histone H3?

NEQ288, one of two archaeal histones in Nanoarchaeum equitans, has a unique four-residue insertion that closely resembles an insertion in the eukaryotic histone H3 lineage. NEQ288 bound DNA but did not compact DNA in vitro in the absence of NEQ348, the second N. equitans archaeal histone. The properties of NEQ288 suggest an intermediate between the archaeal and H3 histone lineages and an evolutionary step toward the now-mandatory assembly of eukaryotic histones into heterodimers.

An archaeal histone-like protein mediates efficient p53 gene transfer and facilitates its anti-cancer effect in vitro and in vivo

The improvement of the transfection efficiency of the non-viral-based gene delivery systems is a key issue for the application in gene therapy. We have previously described an archaeal histone-like protein-based (HPhA) gene delivery system and showed that HPhA formed stable non-covalent complexes with nucleic acids and improved their delivery by using beta-galactosidase as a reporter gene. In this study, the wild-type p53 gene was transfected into the cancer cells using the HPhA as a vector, and the expression level and the activity of p53 gene were evaluated both in vitro and in vivo. Gene expression was determined by real-time reverse transcriptase-PCR and western blotting analysis. The cellular growth inhibition and apoptosis of HPhA-mediated p53 transfection were assessed by XTT (sodium 3'-[1-(phenylaminocarbonyl)-3,4-tetrazolium]-bis(4-methoxy-6-nitro)benzene sulfonic acid hydrate) assay and annexin V-FITC (fluorescein isothiocyanate) staining, respectively. Further more, transfection of HPhA/p53 into CNE (nasopharyngeal carcinoma cell line)-xenografted nude mice was performed and tumor growth was measured. The present study demonstrates that HPhA enhances the efficiency of p53 gene transfer and antitumor activity compared with the widely used Lipofectamine. These results demonstrate that HPhA enhances the in vitro and in vivo efficiency of p53 gene transfer and suggest that it may be served as a promising tool for gene delivery and gene therapy.

Structural basis of the substrate subsite and the highly thermal stability of xylanase 10B from Thermotoga maritima MSB8

The crystal structure of xylanase 10B from Thermotoga maritima MSB8 (TmxB), a hyperthermostable xylanase, has been solved in its native form and in complex with xylobiose or xylotriose at 1.8 A resolution. In order to gain insight into the substrate subsite and the molecular features for thermal stability, we compared TmxB with family 10 xylanase structures from nine microorganisms. As expected, TmxB folds into a (beta/alpha)8-barrel structure, which is common among the glycoside hydrolase family 10. The enzyme active site and the environment surrounding the xylooligosaccharide of TmxB are highly similar to those of family 10 xylanases. However, only two xylose moieties were found in its binding pocket from the TmxB-xylotriose complex structure. This finding suggests that TmxB could be a potential biocatalyst for the large-scale production of xylobiose. The result of structural analyses also indicated that TmxB possesses some additional features that account for its thermostability. In particular, clusters of aromatic residues together with a lack of exposed hydrophobic residues are characteristic of the TmxB structure. TmxB has also a significant number of ion pairs on the protein surface that are not found in other thermophilic family 10 xylanases.

Asymmetry in the burial of hydrophobic residues along the histone chains of eukarya, archaea and a transcription factor

Background

The histone fold is a common structural motif of proteins involved in the chromatin packaging of DNA and in transcription regulation. This single chain fold is stabilized by either homo- or hetero-dimer formation in archaea and eukarya. X-ray structures at atomic resolution have shown the eukaryotic nucleosome core particle to consist of a central tetramer of two bound H3-H4 dimers flanked by two H2A-H2B dimers. The c-terminal region of the H3 histone fold involved in coupling the two eukaryotic dimers of the tetramer, through a four-fold helical bundle, had previously been shown to be a region of reduced burial of hydrophobic residues within the dimers, and thereby provide a rationale for the observed reduced stability of the H3-H4 dimer compared with that of the H2A-H2B dimer. Furthermore, comparison between eukaryal and archaeal histones had suggested that this asymmetry in the distribution of hydrophobic residues along the H3 histone chains could be due to selective evolution that enhanced the coupling between the eukaryotic dimers of the tetramer.

Results and discussion

The present work describes calculations utilizing the X-ray structures at atomic resolution of a hyperthermophile from Methanopyrus kandleri (HMk) and a eukaryotic transcription factor from Drosophila melanogaster (DRm), that are structurally homologous to the eukaryotic (H3-H4)₂ tetramer. The results for several other related structures are also described. Reduced burial of hydrophobic residues, at the homologous H3 c-terminal regions of these structures, is found to parallel the burial at the c-terminal regions of the H3 histones and is, thereby, expected to affect dimer stability and the processes involving histone structural rearrangement. Significantly different sequence homology between the two histones of the HMk doublet with other archaeal sequences is observed, and how this might have occurred during selection to enhance tetramer stability is described.

How Oligomerization Contributes to the Thermostability of an Archaeon Protein

To study how oligomerization may contribute to the thermostability of archaeon proteins, we focused on a hexameric protein, protein L-isoaspartyl-O-methyltransferase from Sulfolobus tokodaii (StoPIMT). The crystal structure shows that StoPIMT has a distinctive hexameric structure composed of monomers consisting of two domains: an S-adenosylmethionine-dependent methyltransferase fold domain and a C-terminal alpha-helical domain. The hexameric structure includes three interfacial contact regions: major, minor, and coiled-coil. Several C-terminal deletion mutants were constructed and characterized. The hexameric structure and thermostability were retained when the C-terminal alpha-helical domain (Tyr(206)-Thr(231)) was deleted, suggesting that oligomerization via coiled-coil association using the C-terminal alpha-helical domains did not contribute critically to hexamerization or to the increased thermostability of the protein. Deletion of three additional residues located in the major contact region, Tyr(203)-Asp(204)-Asp(205), led to a significant decrease in hexamer stability and chemico/thermostability. Although replacement of Thr(146) and Asp(204), which form two hydrogen bonds in the interface in the major contact region, with Ala did not affect hexamer formation, these mutations led to a significant decrease in thermostability, suggesting that two residues in the major contact region make significant contributions to the increase in stability of the protein via hexamerization. These results suggest that cooperative hexamerization occurs via interactions of "hot spot" residues and that a couple of interfacial hot spot residues are responsible for enhancing thermostability via oligomerization.

Recombinant expression and characterization of an extremely hyperthermophilic archaeal histone from Pyrococcus horikoshii OT3

A histone-like gene, PHS051 from hyperthermophilic archaeon Pyrococcus horikoshii OT3 strain, was cloned, sequenced, and expressed in Escherichia coli. The recombinant histone, HPhA, encodes a protein of 70 amino acids with a molecular weight of 7868Da. Amino acid sequence analysis of HPhA showed high homology with other archaeal histones and eukaryal core histones. The HPhA was purified to homogeneity by heat precipitation and affinity chromatography. Gel electrophoresis mobility shift assays demonstrate that the purified HPhA has high affinity to DNA. The complex of the HPhA and DNA allows DNA to be protected from cleavage by the restriction enzyme TaqI at 65 degrees C. Circular dichroism spectra reveal that the conformation of the recombinant histone HPhA becomes looser when temperatures increase from 25 to 90 degrees C. The HPhA has inherited a remarkable thermostability especially in the presence of 1M KCl and retained DNA binding activity at extreme temperature, which is consistent with our previous report about its structure stability analyzed by X-ray crystallography.

Aromatic side-chain interactions in proteins: Near- and far-sequence Tyr-X pairs

In the present study, an extensive analysis of the aromatic Tyr-X interactions is performed on a data set of 593 PDB structures, X being Phe, His, Tyr, and Trp. The nonredundant Tyr-X pairs (2645) were retained and separated by both the residue distance in the sequence and the secondary structures they bridge. Similar to the Phe-X and His-X pairs, the far-sequence Tyr-X pairs (X partner > five apart in the sequence: 74%) show comparable secondary structures and conformers for either type of X partner, in contrast with the near-sequence Tyr-X pairs (26%). As the Phe-X pairs, the near-sequence Tyr-X pairs stabilize secondary structures, mainly the alpha- helices (positions 1, 3, and 4) and the beta-strands (position 2). Like the Phe-X and His-X pairs, most far-sequence Tyr-X pairs (34%) bridge beta-strands and only 11% bridge helices. As for the Phe-X and the His-X pairs, the X partners of the far-sequence Tyr-X pairs are frequently "above" the tyrosine ring with tilted and normal rings, whereas the X partner of the near-sequence Tyr-X pairs gradually moves from the "aside" to the "above" location, together with a progressive decrease of normal and increase of parallel rings, respectively. Unlike the His-X pairs, the interactions of the hetroatom in Tyr-X pairs are only favored with a sequence position +4 and over, owing to the spatial accessibility of the heteroatom.

Metabolic integration during the evolutionary origin of mitochondria

Although mitochondria provide eukaryotic cells with certain metabolic advantages, in other ways they may be disadvantageous. For example, mitochondria produce reactive oxygen species that damage both nucleocytoplasm and mitochondria, resulting in mutations, diseases, and aging. The relationship of mitochondria to the cytoplasm is best understood in the context of evolutionary history. Although it is clear that mitochondria evolved from symbiotic bacteria, the exact nature of the initial symbiosis is a matter of continuing debate. The exchange of nutrients between host and symbiont may have differed from that between the cytoplasm and mitochondria in modern cells. Speculations about the initial relationships include the following. (1) The pre-mitochondrion may have been an invasive, parasitic bacterium. The host did not benefit. (2) The relationship was a nutritional syntrophy based upon transfer of organic acids from host to symbiont. (3) The relationship was a syntrophy based upon H2 transfer from symbiont to host, where the host was a methanogen. (4) There was a syntrophy based upon reciprocal exchange of sulfur compounds. The last conjecture receives support from our detection in eukaryotic cells of substantial H2S-oxidizing activity in mitochondria, and sulfur-reducing activity in the cytoplasm.

New understandings of thermostable and peizostable enzymes

Recent large-scale studies illustrate the importance of electrostatic interactions near the surface of proteins as a major factor in enhancing thermal stability. Mutagenesis studies have also demonstrated the importance of optimized charge interactions on the surface of the protein, which can significantly augment enzyme thermal stability. Directed evolution studies show that increased stability may be obtained by different routes, which may not mimic those used by nature. Despite observations that some of the most thermotolerant organisms grow under conditions of high pressure, little effort has been made to understand the correlation between pressure and temperature stability. One recent study demonstrates that the active-site volume may be important in increasing pressure stability.