SV40 large T antigen
SV40 large T antigen (Simian Vacuolating Virus 40 TAg) is a hexamer protein that is a proto-oncogene derived from the polyomavirus SV40 which is capable of transforming a variety of cell types. The transforming activity of TAg is due in large part to its perturbation of the retinoblastoma (pRB) and p53 tumor suppressor proteins. In addition, TAg binds to several other cellular factors, including the transcriptional co-activators p300 and CBP, which may contribute to its transformation function.
TAg is a product of an early gene transcribed during viral infection by SV40, and is involved in viral genome replication and regulation of host cell cycle. SV40 is a double-stranded, circular DNA virus belonging to the Polyomaviridae (earlier Papovavirus) family, Orthopolyomavirus genus. Polyomaviruses infect a wide variety of vertebrates and cause solid tumours at multiple sites. SV40 was isolated by Sweet and Maurice Hilleman in 1960 in primary monkey kidney cell cultures being used to grow Sabin OPV.
The genome is functionally divided into 3 regions:
- Early: Expressed early in virus infection, i.e. BEFORE genome replication. Expression of early genes continues during the late stage of infection. Encodes non-structural proteins (i.e. not present in virus particle).
- Late: Expressed later in virus infection, i.e. DURING & AFTER genome replication. Encodes structural proteins (i.e. present in virus particle).
- Regulatory region: Contains transcriptional promoters & enhancers plus the unique origin of DNA replication.
After entering the cell, the viral genes are transcribed by host cell RNA polymerase II to produce early mRNAs. Because of the relative simplicity of the genome, polyomaviruses are heavily dependent on the cell for transcription and genome replication. The cis-acting regulatory element surrounding the origin of replication directs transcription, and T-antigen directs transcription and replication.
SV40 DNA replication is initiated by binding of large T-antigen to the origin region of the genome. The function of T-antigen is controlled by phosphorylation, which attenuates the binding to the SV40 origin. Protein-protein interactions between T-antigen and DNA polymerase-alpha directly stimulate replication of the virus genome.
The SV40 genome is very small and does not encode all the information necessary for DNA replication. Therefore, it is essential for the host cell to enter S phase, when cell DNA and the viral genome are replicated together. Therefore, in addition to increasing transcription, another function of T-antigen is to alter the cellular environment to permit virus genome replication.
Nuclear localization signal
The SV40 large T-antigen has been used as a model protein to study nuclear localization signals (NLSs). It is imported into the nucleus by its interaction with importin α. The NLS sequence is PKKKRKV.
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