Identification of an Amsacta spheroidin-like protein within the occlusion bodies of Choristoneura entomopoxviruses.

Like baculoviruses, a characteristic feature of entomopoxviruses is the amalgamation of virions within environmentally stable occlusion bodies. It is this occluded form of the virus that is primarily responsible for dissemination to other insects. While the major protein (polyhedrin) of baculovirus occlusions is quite similar between viruses, it has been reported that the major occlusion body protein (spheroidin) of two group B entomopoxviruses, Amsacta moorei (AmEPV) and Choristoneura biennis (CbEPV) is quite different both in terms of amino acid sequence and coding capacity of the corresponding spheroidin genes (115 and 47 kDa for AmEPV and CbEPV, respectively). We report the discovery of a AmEPV spheroidin gene homolog in both CbEPV and a second Choristoneura virus, Choristoneura fumiferana (CfEPV). Antibodies directed against the AmEPV 115-kDa spheroidin reacted with the major protein of approximately 115 kDa found within the occlusion body preparation from both Choristoneura viruses. Direct protein microsequencing of small portions of the 115-kDa protein from CbEPV has resulted in peptide sequences identical to those of corresponding regions of the AmEPV spheroidin gene. We suggest that it is this Choristoneura gene which encodes spheroidin. All attempts, however, to find a homolog of the previously reported CbEPV spheroidin gene within AmEPV have been unsuccessful. We also show this newly identified Choristoneura homolog of the AmEPV spheroidin gene as well as the AmEPV spheroidin gene itself are both located at the 3' end of an NPH I gene and are highly homologous in all three viruses, indicating that this region of the genome in the three viruses is co-linear. These results and others suggest that while the insect viruses lack the traditional central core of conserved genes observed for the vertebrate poxviruses, the insect poxviruses may have also evolved an alternative central core of conserved genes, unique to the invertebrate poxviruses.