Early phase in the infection of cultured cells with papillomavirus virions.

The fate of full bovine papillomavirus (BPV) virions and virus-like particles after binding to C127 or CV-1 cells was studied by electron microscopy and indirect immunofluorescence. After incubation at 4 degrees for 1 hr, BPV virions were found to be bound to the plasma membrane, and most viruses were absorbed by the cells after 30 min incubation at 37 degrees. Ninety minutes after the virions had been bound to the plasma membrane, the uptake of the virions was completed and most of the antigen was found to be localized in the nucleus. The viruses were transported in phagosomes and the uptake and transportation could be inhibited by cytochalasin B and taxol, suggesting the possible involvement of microfilaments and microtubules in the virus particle uptake and transportation. The capsid proteins could be detected for about 14 hr, until degradation and deposit of the viral antigen in the Golgi complexes. Although binding to the plasma membrane and uptake of virions into large cytoplasmic vesicles could be monitored by electron microscopy, no complete virions were observed in the nucleus of infected cells despite a very strong nuclear fluorescent staining for both L1 and L2 proteins. This may indicate that disintegration of the virions occurs in the cytoplasm and the L1/L2 proteins migrate to the nucleus via their nuclear localization signals.