SV40 Large T-antigen
Encyclopedia
SV40 large T antigen 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 DNA virus, belongs to Polyomaviridae (earlier Papovavirus
) family, Polyomavirus
genus. Polyomaviruses infect a wide variety of vertebrates and it caused solid tumours at multiple sites. SV40 was isolated by Sweet and Maurice Hilleman
in 1960 in primary monkey kidney cells cultures being used to grow Sabin
OPV.
Polyomavirus
genomes are double-stranded, circular DNA molecules, and are approximately 5 kilo-base pair
s in length.
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.
T-antigen also binds and inactivates tumor suppressor proteins (p53, p105). This causes the cells to leave G1 phase and enter into S phase, which promotes DNA replication
.
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 virus 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.
. The NLS sequence is PKKKRKV.
Polyomavirus
Polyomavirus is the sole genus of viruses within the family Polyomaviridæ. Murine polyomavirus was the first polyomavirus discovered by Ludwik Gross in 1953. Subsequently, many polyomaviruses have been found to infect birds and mammals...
SV40
SV40
SV40 is an abbreviation for Simian vacuolating virus 40 or Simian virus 40, a polyomavirus that is found in both monkeys and humans...
which is capable of transforming
Transformation (genetics)
In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...
a variety of cell types. The transforming activity of TAg is due in large part to its perturbation of the retinoblastoma
Retinoblastoma
Retinoblastoma is a rapidly developing cancer that develops in the cells of retina, the light-detecting tissue of the eye. In the developed world, Rb has one of the best cure rates of all childhood cancers , with more than nine out of every ten sufferers surviving into...
(pRB
Retinoblastoma protein
The retinoblastoma protein is a tumor suppressor protein that is dysfunctional in the majority types of cancer. One highly studied function of pRb is to prevent excessive cell growth by inhibiting cell cycle progression until a cell is ready to divide...
) and p53
P53
p53 , is a tumor suppressor protein that in humans is encoded by the TP53 gene. p53 is crucial in multicellular organisms, where it regulates the cell cycle and, thus, functions as a tumor suppressor that is involved in preventing cancer...
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 DNA virus, belongs to Polyomaviridae (earlier Papovavirus
Papovavirus
A Papovavirus is any member of the former virus family of Papovaviridae. They are mainly associated with various neoplasms in mammals. The family of Papovaviridae is not longer used in recent taxonomy, but is split into the Papillomaviridae and the Polyomaviridae.The name derives from three...
) family, Polyomavirus
Polyomavirus
Polyomavirus is the sole genus of viruses within the family Polyomaviridæ. Murine polyomavirus was the first polyomavirus discovered by Ludwik Gross in 1953. Subsequently, many polyomaviruses have been found to infect birds and mammals...
genus. Polyomaviruses infect a wide variety of vertebrates and it caused solid tumours at multiple sites. SV40 was isolated by Sweet and Maurice Hilleman
Maurice Hilleman
Maurice Ralph Hilleman was an American microbiologist who specialized in vaccinology and developed over three dozen vaccines, more than any other scientist...
in 1960 in primary monkey kidney cells cultures being used to grow Sabin
Albert Sabin
Albert Bruce Sabin was an American medical researcher best known for having developed an oral polio vaccine.-Life:...
OPV.
Polyomavirus
Polyomavirus
Polyomavirus is the sole genus of viruses within the family Polyomaviridæ. Murine polyomavirus was the first polyomavirus discovered by Ludwik Gross in 1953. Subsequently, many polyomaviruses have been found to infect birds and mammals...
genomes are double-stranded, circular DNA molecules, and are approximately 5 kilo-base pair
Base pair
In molecular biology and genetics, the linking between two nitrogenous bases on opposite complementary DNA or certain types of RNA strands that are connected via hydrogen bonds is called a base pair...
s in length.
Regions
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.
Mechanism
After entering the cell, the virus genes are transcribed by host cell RNA polymerase IIRNA polymerase II
RNA polymerase II is an enzyme found in eukaryotic cells. It catalyzes the transcription of DNA to synthesize precursors of mRNA and most snRNA and microRNA. A 550 kDa complex of 12 subunits, RNAP II is the most studied type of RNA polymerase...
to produce early mRNAs. Because of the relative simplicity of the genome, polyomaviruses are heavily dependent on the cell for transcription and genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
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
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
. The function of T-antigen is controlled by phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
, 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.
T-antigen also binds and inactivates tumor suppressor proteins (p53, p105). This causes the cells to leave G1 phase and enter into S phase, which promotes DNA replication
DNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...
.
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
S phase
S-phase is the part of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Precise and accurate DNA replication is necessary to prevent genetic abnormalities which often lead to cell death or disease. Due to the importance, the regulatory pathways that govern this...
, when cell DNA and the virus 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 model to protein to study nuclear localisation signals (NLSs). It is imported into the nucleus by its interaction with importin αImportin α
Importin alpha, or karyopherin alpha proteins are involved in the import of proteins into the cell nucleus. They are a sub-family of karyopherin proteins....
. The NLS sequence is PKKKRKV.