MPyVgp5

New MPyV virions are assembled in the nucleus in dense local aggregations known as virus factories. Capsid proteins, produced in the cytoplasm of the host cell, enter the nucleus as assembled capsomers consisting of pentameric VP1 associated with VP2 or VP3. Nuclear localization sequences consistent with karyopherin interactions have been identified in capsid protein sequences, facilitating their transit through nuclear pores. Once inside the nucleus they assemble into mature capsids containing a copy of the viral genome, although the exact mechanism of encapsidation is not well understood. Filamentous or tubular structures representing polymerized VP1 have been observed in the nuclei of infected cells as intermediates in the assembly process from which mature virions are produced.