The predicted amino acid sequence of the spheroidin protein from Amsacta moorei entomopoxvirus: Lack of homology between major occlusion body proteins of different poxviruses

Entomopoxviruses replicate in the cytoplasm of insect cells and characteristically produce occlusion bodies which serve to protect the virion from the environment; the major component of these bodies is a protein called spheroidin. We have previously identified and sequenced the gene encoding the major occlusion body protein of eastern spruce budworm (Choristoneura biennis) entomopoxvirus (CbEPV) and found it to encode a 47K polypeptide which aggregates due to the formation of intermolecular disulphide bonds. In this publication we demonstrate that the insect poxvirus of Amsacta moorei produces spheroidin with a unit M(r) of 114.8K. The gene for this protein was cloned and sequenced and the predicted polypeptide was demonstrated to contain 38 cysteine residues a leucine zipper for possible protein-protein interactions and 14 potential Asn-linked glycosylation sites. Other than possessing a large number of sulphydryl groups, this protein showed no homology to its analogue found in cell infected with CbEPV. Antibodies directed against occlusion body proteins of the two viruses also failed to cross-react significantly on Western blots. In addition, nucleic acid probes prepared from the two different genes did not cross-hybridize on Southern blots of genomic DNA prepared from the viruses. Finally, the occlusion body proteins from the two insect viruses were compared with the A-type inclusion body protein of cowpox virus. Again, little homology between these proteins was evident, with the exception of a generally high cysteine content and a similarity between their late gene promoters. We conclude that the major occlusion body proteins of different poxviruses possess diverse primary structures, but all are capable of yielding large aggregates through the formation of disulphide bonds.