Avian polyomavirus major capsid protein VP1 interacts with the minor capsid proteins and is transported into the cell nucleus but does not assemble into capsid-like particles when expressed in the baculovirus system

The baculovirus system was used to construct and isolate AcMNPV-VP1, AcMNPV -VP2 and AcMNPV-VP3 recombinant viruses which express the respective avian polyomavirus (APV) structural proteins in Sf9 insect cells. These recombina nt AcMNPVs containing APV structural protein genes were utilized to investi gate protein-protein interactions between the structural proteins. Immunofl uorescence studies utilizing Sf9 cells infected with the AcMNPV-VP1 reveale d that the VPI protein was expressed and localized in the cytoplasm and not transported into the nucleus. When the cells were co-infected with the VPI and either VP2 or VP3 recombinant viruses, immunofluorescence of the VPI p rotein was localized in the nucleus, indicating that the VP1 protein was tr ansported to the nucleus by both the VP2 and VP3 minor proteins. This obser vation was suggestive of a protein-protein interaction between the expresse d proteins. This protein-protein interaction was substantiated by laser sca nning confocal microscopy of Sf9 cells that were co-infected with VP1, VP2 and VP3 recombinant viruses. However, the minor proteins could not be co-is olated with VPI protein by immunoaffinity chromatography using a monoclonal anti-VP1 serum. In addition, capsid-like particles could not be purified e ither by CsCl density gradient centrifugation or by immunoaffinity chromato graphy. VP1 capsomeres were isolated by immunoaffinity chromatography from Sf9 cells infected with AcMNPV-VP1, with or without the minor protein(s), a nd these capsomeres could assemble in vitro into capsid-like particles. Ele ctron microscopic observation of thin-sectioned Sf9 cells, which were co-in fected with VP1, VP2 and VP3 recombinant viruses, demonstrated capsomere-li ke structures in the nucleus, but capsid-like particles were not observed. (C) 1999 Elsevier Science B.V. All rights reserved.