The spheroidin of an Entomopoxvirus isolated from the grasshopper Anacridiumaegyptium (AaEPV) shares low homology with spheroidins from lepidopteran or coleopteran EPVs

Abstract

Based on virion morphology, the current virus taxonomy groups entomopoxviruses (EPVs) (Poxvirus: Entomopoxvirinae) from coleopteran and dipteran hosts in separated genera, wilts it keeps viruses infecting either lepidopteran or orthopteran hosts in the same genus. In contrast to the morphological criteria, the few data available from recent studies at the genetic level have suggested that EPVs infecting different insect orders are phylogenetically distant. In order to elucidate EPVs phylogeny we have cloned and sequence the highly conserved/highly expressed spheroidin gene of Anacridium aegyptium entomopoxvirus (AaEPV). This gene and its promoter is of interest for the development of genetic engineering on EPVs. The spheroidin gene was located in the AaEPV genome by Southern blot and hybridisation with specific degenerated oligonucleotides probes synthesised after partial sequencing of the purified spheroidin protein. A total of 3489 bp were sequenced. This sequence included the coding and promoter region of 969 residues 108.8 kDa protein identified as spheroidin. AaEPV spheroidin contains 21 cysteine residues (2.2%) and 14 N-glycosylation putative sites distributed along the sequence. The cysteine residues are particularly abundant at the C-terminal end of the protein, with 11 residues in the last 118 aa. Our results confirm that the spheroidin is highly conserved only between EPVs isolated from the same insect order. Polyclonal antibodies raised against AaEPV spherules specifically revealed spheroidin in Western Blots failing to cross-react with MmEPV or AmEPV spheroidins or MmEPV fusolin. Comparison of spheroidins at the aa level demonstrate that AaEPV spheroidin shares only 22.2 and 21.9% identity with the lepidopteran AmEPV and the coleopteran MmEPV spheroidins, respectively, but 82.8% identity with the orthopteran MsEPV spheroidin. Only two highly conserved domains containing the sequence (V/Y)NADTG(C/L) and LFAR(I/A) have been identified in all known spheroidins. The phylogenetic tree constructed according to the clustalx analysis program revealed that EPVs are clearly separated in three groups — lepidopteran, coleopteran and orthopteran — according to the insect order of the virus hosts. In base to our results, the split of the genus Entomopoxvirus B dissociating lepidopteran and orthopteran EPVs into two different genera is suggested.