Association of a baculovirus-encoded protein with the capsid basal region

Abstract

An open reading frame homologous to AcMNPV ORF9 (ORF1629) was characterized in the Orgyia pseudotsugata multinucleocapsid nuclear polyhedrosis virus (OpMNPV). Sequence analysis indicated that the OpMNPV homolog (called ORF2) encoded a protein predicted to contain 545 amino acids with a molecular weight of 61 kDa. The first 80 amino acids did not have a counterpart in the AcMNPV homolog. The remainder of the ORF was poorly conserved with 29% amino acid identity overall with the AcMNPV ORF. However, the amino terminal 150 amino acids of AcMNPV ORF9 demonstrated about 45% amino acid sequence identity with OpMNPV ORF2 and conserved runs of proline residues were present in internal regions of both molecules. Transcriptional mapping indicated the ORF2 transcripts were initiated at a late promoter sequence, ATAAG, beginning about 24 hr p.i. These transcripts terminated near the 3' end of the ORF2 reading frame. Antibodies were produced against a fusion protein derived from the bacterial gene encoding the maltose binding protein and most of the ORF2 sequence. These antibodies reacted with a protein of 69 kDa on Western blots and the protein was found to be associated with virions isolated from both polyhedra and budded virus. The OpMNPV ORF2 antiserum also reacted with the AcMNPV ORF9 gene product. Immunoelectron microscopic analyses indicated that ORF2 was associated with the ends of the capsids which contain the basal structure. This end appears to be oriented away from both the virogenic stroma and membranes involved in intranuclear envelopment. In addition, as virions bud from the nucleus into the cytoplasm, this end also appears to be oriented away from the nuclear membrane.