Identification and characterization of a baculovirus structural protein, VP1054, required for nucleocapsid formation.

The defect in a temperature-sensitive mutant of Autographa californica nuclear polyhedrosis virus, tsN1054, was mapped and characterized. At the nonpermissive temperature of 33 degrees C, this mutant fails to form plaques upon infection of Sf-21 cultured insect cells; infection is limited to a single cell, even though the infection proceeds through the very late phase. Marker rescue mapping and DNA sequencing identified the gene, ORF 54, which was altered by a single nucleotide substitution in tsN1054. Transcriptional analysis of the ORF 54 region identified multicistronic RNAs, from early to very late times of infection, that potentially encode the ORF 54 gene product. Polyclonal antiserum raised to a TrpE-VP1054 fusion protein recognized a 42-kDa late protein, VP1054, in infected-cell lysates. VP1054 was found to be a component of both budded virus and occlusion-derived virions. The level of VP1054 was dramatically reduced in tsN1054-infected Sf-21 cells propagated at 33 degrees C, and electron microscopic analysis of these cells showed that nucleocapsids failed to form in the nuclei of these infected cells. Instead, novel round, electron-dense bodies were found associated with the virogenic stroma in tsN1054-infected cells. Therefore, VP1054 is a virus structural protein required for nucleocapsid assembly.