Expression of major capsid protein VP-1 in the absence of viral particles in thymomas induced by murine polyomavirus

Thymomas induced by polyomavirus strain PTA in mice are known to express th e major capsid protein VP-1. Since the expression of a late structural prot ein such as VP-1 is considered a sign of virus replication, the present wor k attempted to clarify the implication of the presence of this protein in t umor cells. Electron microscopy of tumors showed a striking absence of vira l particles in the vast majority of the cells. However, immunoelectron micr oscopy of the same samples demonstrated intranuclear VP-1 in most cells des pite the absence of viral particles. Very little infectious virus was recov ered from tumors. A change in the electro phoretic mobility of VP-1 from th ymomas was detected compared with VP-1 from productively infected cells. Th e data presented in this work prove that the expression of VP-1 in polyomav irus-induced tumors is not synonymous with the presence of infectious virus , suggesting a possible defect in viral encapsidation.