Polyomavirus T Antigens: Molecular Chaperones for Multiprotein Complexes

serve as powerful tools to identify key cellular regulatory pro-teins and to discern the mechanisms by which these proteins exert their biological effects. Like all members of the polyoma-virus group, SV40 and PyV have simple genomes (Fig. 1) that can be divided into an early region that is expressed prior to the onset of viral DNA replication and encodes the viral tumor antigens (T antigens) and a late region that encodes the viral capsid proteins (VP1, VP2, and VP3) (for a review, see refer-ence 6). For the most part, SV40 and PyV co-opt cellular machines for viral DNA replication and transcription. In cell culture systems, the first 20 h postinfection is dedicated to driving the infected cells into the cell cycle so that cellular proteins needed for viral replication are expressed and in re-directing cell macromolecular synthesis systems to function in viral replication, transcription, and virion assembly. During