Evolutionary Trace Analysis Of The Precursor Polyprotein Encoded By The Genome Segment A Of Infectious Bursal Disease Virus

Divergence of virus virulence has been one of the focuses in the animal virology, specifically manystudies have attempted to elucidate the molecular basis of virulence divergence of infectious bursaldisease virus. Previous studies have shown that the precursor polyprotein where VP2is located has ahigher mutation frequency than other regions of the IBDV genome, even it can be argued that thisregion can be used to distinguish the variant strains of IBDV, such as variant, classic and very virulentstrains. Therefore the study based on the precursor polyprotein can provide more valuable informationwith regard to IBDV virulence variation than other relatively conservative region.With the development of cross-area application of Structural Biology, Genomics and Bio-physics andmany other disciplines, it is possible to study the structure and function of biological molecules fromsystematic evolution point of view. To study the relation between IBDV amino acid sequences and itsvirulence divergence, this study collected28amino acid sequences of precursor polyprotein from IBDVstrains of three different virulent level, obtained from the Genbank, using evolutionary traceanalysis(ETA) method to carry out phylogenetic analysis, then mapping the specific and distinct sites onthe simulated protein structure built by the computer with a attempt to explain the possible causes fordivergence of IBDV virulence.The study found that most of amino acid residues conserved in each virulent-level group concentrated inthe VP2protein, and it may be due to the fact that VP2as one of the major structural proteins of IBDV,located on the membrane of the virus, it plays an important role in the initial stages of the virus infectionof susceptible cells, such as recognition of and adhesion to host cell receptors. This may lead to theconclusion that amino acid mutations in the VP2protein may be the reason for cell tropism andvirulence divergence. Specifically, the distinct amino acids between very virulent IBDV strains(vvIBDV)and classic IBDV strains (cIBDV) reside in the positions of242,253,294,299,451,680,715,751of theamino acids sequences of precursor polyprotein; those between the attenuated IBDV strains (atIBDV)and cIBDV reside in the positions of242253,330,981. Especially the253,299and330positions’mutation are consistent with published experimental results. Our analysis of these mutations’ impacts is:cIBDV and atIBDV’s299N is replaced by vvIBDV’s S, S having hydrogen bonds could enhance theintra-molecular or intermolecular interactions, which then may enhance the virulence of vvIBDV; thesame applies for vvIBDV and cIBDV’s330S changes to atIBDV’s330R. In addition,451,680,715and751positions’ mutations were near the VP2-the VP4and the VP4-VP3cleavage sites, indicating thatthese amino acids may be acting on the precursor polyprotein processing and change the virusreplication rate in host cell. P981L replacement may affect VP3formation, but this needs furtherexperimental confirmation.Further, based on the former evolutionary trace analysis results this study built a VP2protein model of a very virulent strain UK661, then mapping the amino acids evolutionary residues of vvIBDV, cIBDVand atIBDV including I242Y、Q253H、I294L、S299N and S330R to the VP2model respectively. Aftercomparing those residues’ sites on the VP2we found: Q253H experienced structural difference fromlinear Q into ring H, which may increase the possibility of β-sheet formation thus leading to theformation of the structural hindrance in VP2which could reduce the intermolecular interactions; S299Nand S330R also exhibit the same effects which may make VP2’s bonding with the host cell receptorweaker; however since I242Y, I294L are in inside of the VP2their mutations might not have importanteffects on IBDV virulence divergence.This study was designed to apply the method of evolutionary trace analysis on the amino acidssequences deduced from the precursor polyprotein encoded by the genome segment A of IBDV in orderto explore the intrinsic link between IBDV amino acids sequences and its virulence divergence, aimingto provide feasible ways and means for future further study of IBDV molecular biologicalcharacteristics and references for the development of more effective IBDV vaccines and diagnosticreagents, which has great significance in effective prevention and control of Infectious Bursal Disease(IBD).