Polyomavirus T Antigens: Molecular Chaperones for Multiprotein Complexes

Simian virus 40 (SV40) and murine polyomavirus (PyV) serve as powerful tools to identify key cellular regulatory proteins and to discern the mechanisms by which these proteins exert their biological effects. Like all members of the polyomavirus group, SV40 and PyV have simple genomes (Fig. ​(Fig.1)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 reference 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 redirecting cell macromolecular synthesis systems to function in viral replication, transcription, and virion assembly. During this period the only viral proteins expressed are the T antigens, which play three critical roles in productive infection. The viral T antigens (i) alter and/or recruit specific host cell proteins to participate in virus production, (ii) block cellular antiviral defense systems, and (iii) participate directly in viral replication (e.g., large T antigens are DNA helicases and are thus required to replicate viral DNA). FIG. 1 Genetic maps of SV40 and PyV. ori indicates position of the origin of DNA replication, PE is the early region promoter, PL is the late region promoter. Arrowed lines indicate coding sequences for viral proteins. Arrowhead indicates the carboxy terminus ... In the polyomaviruses, all the T antigens are encoded by a common precursor mRNA that is differentially spliced to yield multiple monocistronic mature mRNAs. SV40 expresses three such mRNAs, one each for large T antigen, small t antigen, and tiny t antigen, whereas PyV expresses four mRNAs, one each for large, middle, small, and tiny T antigens (Fig. ​(Fig.2).2). The splicing pattern is such that all T antigens encoded by a given virus have a common amino-terminal domain. The large, middle, and small T antigens are multifunctional proteins that carry a diverse array of biochemical activities (6, 27). Some of these activities reside in discrete functional domains, while others require interdomain interaction and/or the independent action of two or more domains. In cell culture, only the large T antigen (SV40) or the large and middle T antigens (PyV) are essential for productive infection and viral tumorigenicity. The small t antigens, while not absolutely essential, contribute to both infection and transformation. Little is known about the tiny T antigens (36, 54). FIG. 2 SV40 early region. Structure of the three early mRNAs is shown, along with a simplified domain map of the viral T antigens. Numbers above the line indicate SV40 genomic nucleotide position. Numbers below the line indicate amino acid position from the ... T antigens mediate most of their biological effects by acting on specific cellular proteins (Table ​(Table1).1). For example, SV40 DNA replication requires the large T-antigen-mediated assembly of a preinitiation complex on the viral origin of replication (ori). T antigen binds ori directly through its DNA-binding domain, after which two T-antigen hexamers associate at ori in an ATP-dependent reaction. T antigen then recruits cellular proteins such as DNA polymerase α, primase, topoisomerase, and RPA to the complex via direct physical associations. Following initiation, each of the hexamers functions in the elongation reaction as a DNA helicase. Transcription control, specifically transactivation of the viral capsid genes and of specific cellular genes, is also mediated by the interaction of large T antigen with cellular transcription factors (9, 18). Finally, the transforming functions of the large, middle, and small t antigens are mediated by their direct physical association with cellular target proteins. For example, large T antigens bind the retinoblastoma protein family of tumor suppressors (pRb, p107, and p130), PyV middle T antigen associates with a number of components of the cellular signal transduction network during transformation, and small t antigen contributes to transformation by acting on the cellular protein phosphatase pp2A (Table ​(Table1).1). Thus, successful infection requires the ordered assembly and rearrangement of several different multiprotein complexes involved in DNA replication, gene expression, and virion assembly. Although the mechanism(s) used by the T antigens to effect these diverse processes has been obscure, recent evidence from a number of laboratories indicates that a class of proteins known as molecular chaperones may coordinate many aspects of polyomavirus infection. TABLE 1 Cellular proteins and protein complexes targeted by viral T antigensa