Cooperation of structural proteins during late events in the life cycle of polyomavirus

The polyomavirus minor late capsid antigen, VP2, is myristylated on its N-terminal glycine, this modification being required for efficient infection of mouse cells. To study further the functions of this antigen, as well as those of the other minor late antigen, VP3, recombinant baculoviruses carrying genes for VP1, VP2, and VP3 have been constructed and the corresponding proteins have been synthesized in insect cells. A monoclonal antibody recognizing VP1, alpha-PyVP1-A, and two monoclonal antibodies against the common region of VP2 and VP3, alpha-PyVP2/3-A and alpha-PyVP2/3-B, have been generated. Reactions of antibodies with antigens were characterized by indirect immunofluorescence, immunoprecipitation, and immunoblot analysis. Immunofluorescent staining of mouse cells infected with polyomavirus showed all antigens to be localized in nuclei. When the late polyomavirus proteins were expressed separately in insect cells, however, only VP1 was efficiently transported into the nucleus; VP2 was localized discretely around the outside of the nucleus, and VP3 exhibited a diffused staining pattern in the cytoplasm. Coexpression of VP2, or VP3, with VP1 restored nuclear localization. Immunoprecipitation of infected mouse cells with either anti-VP1 or anti-VP2/3 antibodies precipitated complexes containing all three species, consistent with the notion that VP1 is necessary for efficient transport of VP2 and VP3 into the nucleus. Purified empty capsid-like particles, formed in nuclei of insect cells coinfected with all three baculoviruses, contained VP2 and VP3 proteins in amounts comparable to those found in empty capsids purified from mouse cells infected with wild-type polyomavirus. Two-dimensional gel analysis of VP1 species revealed that coexpression with VP2 affects posttranslational modification of VP1.