-In silico functional characterization of a double histone fold domain from the Heliothis zea virus 1
© Greco et al; licensee BioMed Central Ltd 2005
Published: 1 December 2005
Histones are short proteins involved in chromatin packaging; in eukaryotes, two H2a-H2b and H3-H4 histone dimers form the nucleosomal core, which acts as the fundamental DNA-packaging element. The double histone fold is a rare globular protein fold in which two consecutive regions characterized by the typical structure of histones assemble together, thus originating a histone pseudodimer. This fold is included in a few prokaryotic histones and in the regulatory region of guanine nucleotide exchange factors of the Sos family. For the prokaryotic histones, there is no direct structural counterpart in the nucleosomal core particle, while the pseudodimer from Sos proteins is very similar to the dimer formed by histones H2a and H2b
The absence of a H3-H4-like histone pseudodimer in the available structural databases prompted us to search for proteins that could assume such fold. The application of several secondary structure prediction and fold recognition methods allowed to show that the viral protein gi|22788712 is compatible with the structure of a H3-H4-like histone pseudodimer. Further in silico analyses revealed that this protein module could retain the ability of mediating protein-DNA interactions, and could consequently act as a DNA-binding domain.
Our results suggest a possible functional role in viral pathogenicity for this novel double histone fold domain; thus, the computational analyses here reported will be helpful in directing future biochemical studies on gi|22788712 protein.
DNA packaging in the nucleus of eukaryotic cells is allowed by the assembly of nucleosomal elements, which are composed by a proteic core particle around which DNA is wrapped. The nucleosomal core comprises eight histones, short basic proteins characterized by a high content of lysine and arginine. Several crystallographic and biochemical studies [1–3] have shown that histone H2a is able to form a stable complex with histone H2b, while the H3 monomer can interact with histone H4. The 3D-structure of histones is characterized by the presence of two or three short alpha-helices flanking a longer helix; each of these helices is typically amphiphilic, and the strong interaction between monomers composing a histone dimer is based on the tight packaging of their hydrophobic surfaces.
The histone fold is not a feature specific for eukaryotic histones only; in fact, this fold is also observed in a group of prokaryotic histones , in some transcription factors , and in the amino-terminal domain of the guanine nucleotide exchange factors of the Sos family . Moreover, the crystallographic analysis of the human homologue of Sos1 (, PDB code 1q9c) and of the prokaryotic histone from Methanopyrus kandleri (, PDB code 1f1e) showed the presence of two different interacting histone fold motifs localized along the same polypeptidic chain. Such a structural arrangement is referred to as "histone pseudodimer" or "double histone fold".
The amino-terminal double histone fold domain of Sos proteins is structurally very similar to the H2a-H2b histone dimer , while for the prokaryotic histone pseudodimer it is not possible to individuate a direct structural counterpart in the eukaryotic nucleosome core particle. Consequently, no H3-H4-like histone pseudodimer has been characterized so far.
Prompted by the above observation, we have searched for new sequences potentially compatible with the structure of a putative H3-H4 histone pseudodimer. The results from this search indicated a viral protein from the Heliothis zea virus 1 (Hzv-1) as a possible H3-H4 double histone fold containing protein; this structural assignment was validated by using several secondary structure prediction and fold recognition methods. Finally, the in silico functional characterization of this histone pseudodimer is reported.
Secondary structure predictions were obtained using three different tools: PSI-Pred , J-pred  and PHD . Meta-predictions were carried out by comparing the results obtained from these three servers, and taking into consideration only the sequence regions that were predicted to assume a particular secondary structure by at least two servers, with a degree of reliability of 50% or higher.
Fold recognition results were obtained using the 3D-jury meta server . The servers used by 3D-jury for consensus building were: 3D-PSSM , Meta-Basic , FFAS03 , FUGUE2 , INUB , and mGenTHREADER .
The Swiss-model server  was used to obtain a 3D-model of the viral histone pseudodimer. The H3-H4 histone dimer from Gallus gallus (PDB code: 1eqz) was chosen as a template. The server generated the model in a fully automatized way, and the reliability of the result from such procedure was checked by means of PROCHEK . The analysis of the model was carried out with Pymol  and Swiss PDB viewer . Swiss PDB-viewer was also used in order to obtain the electrostatic potential map of the histone pseudodimer 3D-model.
The prediction of DNA-binding sites on the H3-H4 histone pseudodimer model was carried with the Pre-Ds server .
Results and discussion
The viral protein gi|22788712 is compatible with a H3-H4-like double histone fold
The absence of known H3-H4-like histone pseudodimers in the available structural databases did not allow to apply a standard PSI-Blast search as a starting point of the present work. Consequently, we applied a specific search strategy based on the submission to Psi-Blast of some "chimeric" sequences obtained linking different protein regions included in the H3 and H4 monomers of the histone dimer from Gallus gallus. In particular, the submission of a query sequence comprising the sequence segments 20–103 and 40–136 from histones H4 and H3 evidenced the existence of a viral protein (NCBI code gi|22788712) from the Heliothis zea virus 1 which encompasses two consecutive regions, respectively homologous to histones H4 and H3. This protein appeared already at the first iteration, and the corresponding E-value (6e-7) underlines the statistical relevance of the match. The gi|22788712 protein includes a long N-terminal module of unknown function, while the regions of homology to histone H4 (residues 905–980) and H3 (residues 990–1095) are localized along the C-terminal part of the aminoacidic sequence. Such viral polypeptide is defined as "histone H3, H4" in the corresponding NCBI record; however, this generic annotation is not sufficient to assign a double histone fold domain to this module. Actually, the formation of a histone pseudodimer is expected to require a strict conservation of hydrophobic patterns and secondary structure elements on both the histone folds ; moreover, the linker region between the two histone folds must be sufficiently long and flexible to allow the assumption of a globular fold. Consequently, we decided to carry out an in silico analysis in order to verify if this viral protein sequence is compatible with the presence of a histone pseudodimer. The computational results we obtained have been also used to propose a functional role for this protein module: in fact, viral proteins comprising histone folds are very rare, and no experimental data on them are available at present.
An analysis based on three different secondary structure prediction servers (PHD, Jpred and Psi-PRED, see methods) was then carried out: the results obtained confirmed the structural conservation of the putative alpha-helices corresponding to those normally included in H3 and H4 histone folds (see figure 1). Moreover, all prediction servers indicated that the linker between the two histone folds in the viral protein is characterized by neither an alpha-helix nor a beta-strand conformation, thus suggesting an extended, random coil conformation for this region; this result was expected because, as mentioned above, in a histone pseudodimer the presence of a flexible spacer is necessary to allow the establishment of intramolecular interactions between the two histone folds.
In order to further validate the hypothesis that the two consecutive H3 and H4 histone folds can pack against each other giving rise to a histone pseudodimer, we submitted the corresponding sequence region from the viral protein to the fold recognition meta-server 3D-jury (see Methods). This meta-predictor indicated the structure of the double histone fold domain from Methanopyrus kandleri as the most suitable to describe the fold of the query sequence. Previous literature data  have shown that 3D-jury scores above 50 correspond to correct structure assignment in over 90% of the cases; as for the viral protein gi|22788712, the score reported by the algorithm was 68.67, well above the threshold that indicates a highly reliable structural assignment.
In silico functional characterization of the viral histone pseudodimer
Double histone fold domains from Methanopyrus kandleri and from Sos proteins have very different biological roles: in fact, the prokaryotic histone pseudodimer is implicated in chromatin packaging , while Sos double histone fold domain is known to exert an inhibitory action towards the Ras-GEF activity expressed by this protein class ; moreover, the cytoplasmic localization of Sos proteins  indicate that they should not exhibit function of DNA-binding factors.
The above observations prompted us to carry out an in silico analysis on the novel double histone fold domain from Hzv-1, in order to suggest a possible biological role for this protein module.
It is known that some DNA-virus genomes are complexed with cellular histones to form a chromatin-like structure inside the virus particle . In view of this observation, and considering the results of the computational study here reported, we hypothesize that the double histone fold domain from Hzv1 could contribute to the packaging and organization of viral DNA in the capsid; however, sequence analysis of the viral histone pseudodimer also suggests a possible direct involvement of this protein domain in viral pathogenicity. In fact, the amino-terminal tails of histones H3 and H4 have a fundamental role in the modulation of histones-DNA interaction; consequently, mutations and deletion in these regions can determine a negative effect on nuclear DNA replication and cell cycle progression [32, 33]; notably, these regions are the less conserved in the viral double histone fold sequence, and the expression of such a DNA binding domain in cells infected by the Hzv-1 could interfere with physiological processes of crucial importance for cell growth. However, on such basis our hypothesis would remain speculative, and future biochemical studies will thus be required for its validation.
The double histone fold is an all-alpha protein fold characterized by the tight interaction between two distinct histone folds belonging to the same peptide chain. Previously, this fold has been recognized only in the guanine nucleotide exchange factors of the Sos family and in a few prokaryotic histones.
Sequence analyses, coupled with results from several secondary structure prediction and fold recognition algorithms, allowed to show that also the viral protein gi|22788712 can be included in the group of proteins containing a double histone fold. Further structure-function relationship studies revealed that the chemical-physical properties of the viral histone pseudodimer are compatible with DNA binding; our in silico results will be helpful in directing targeted biochemical studies aiming at the experimental functional characterization of this interesting viral protein domain.
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