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SV40 large T antigen

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SV40 large T antigen
SV40 T helicase domain hexamer, Simian virus.
Identifiers
OrganismSimian virus 40
Symbol?
UniProtP03070
Search for
StructuresSwiss-model
DomainsInterPro

SV40 large T antigen(Simian Vacuolating Virus 40TAg) is ahexamerprotein that is a dominant-actingoncoproteinderived from thepolyomavirusSV40.TAg is capable of inducingmalignant transformationof a variety of cell types. The transforming activity of TAg is due in large part to its perturbation of theretinoblastoma(pRb)[1]andp53tumor suppressor proteins.[2]In addition, TAg binds to several other cellular factors, including the transcriptional co-activatorsp300 and CBP,which may contribute to its transformation function.[3]Similar proteins from related viruses are known aslarge tumor antigenin general.

TAg is a product of an early gene transcribed during viral infection by SV40, and is involved inviral genome replicationand regulation of host cell cycle. SV40 is adouble-stranded,circularDNA virusbelonging to thePolyomaviridae(earlierPapovavirus) family,Orthopolyomavirusgenus. Polyomaviruses infect a wide variety ofvertebratesand cause solid tumours at multiple sites. SV40 was isolated by Sweet andMaurice Hillemanin 1960 in primary monkey kidney cell cultures being used to growSabinOPV.[4]

Domains

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The TAg has aCUL7-binding domain, aTP53-binding domain, a Zinc finger, and a Superfamily 3 ATPase/Helicase domain. It has two motifs, one for nuclear localization signal, the other being the LXCXE motif.[5]

Mechanism

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After entering the cell, the viral genes are transcribed by host cellRNA polymerase IIto produce earlymRNAs.Because of the relative simplicity of the genome, polyomaviruses are heavily dependent on the cell for transcription andgenomereplication. Thecis-actingregulatory 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 thegenome.The function of T-antigen is controlled byphosphorylation,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 inactivatestumor suppressorproteins (p53, p105-Rb). This causes the cells to leave G1 phase and enter into S phase, which promotesDNA 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 enterS 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

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The SV40 large T-antigen has been used as a model protein to studynuclear localization signals(NLSs).[6]It is imported into the nucleus by its interaction withimportin α.[7]The NLS sequence is PKKKRKV.[6]

Interaction with pRb via the LXCXE motif

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SV40 large TAg, otherpolyomaviruslarge T antigens,adenovirusE1a proteins, andoncogenichuman papillomavirusE7 proteins share a structural motif that encodes a high-affinitypRb-binding domain.[8][9]A diagnostic pattern for a high-affinity pRb-binding domain was refined using anartificial intelligencepattern-induction program running on a massively parallelsupercomputer(Connection Machine-2).[9]The motif is characterized by anAsp,AsnorThrresidue followed by three invariant amino acids, interspersed with non-conserved amino acids (designated by x, where x cannot be aLysorArgresidue).[9]A negatively charged region frequently follows carboxy-terminal to the pRb-binding domain.[9]

{Asp/Asn/Thr} –Leu– x –Cys– x –Glu– x –... {negatively charged region}

Hydrophobicandelectrostaticproperties are highly conserved in this motif. For example, a local hydrophobicity maximum occurs in the vicinity of the invariantLeuresidue.[9]A net negative charge occurs within 3 residues amino-terminal to the invariantLeuresidue; furthermore, positively charged amino acids (LysorArg) are not found within theLeu– x –Cys– x –Glusequence, nor in the positions immediately flanking this sequence.[9]The pRb-binding motif and negatively charged region match to a segment of SV40 TAg beginning at residue 102 and ending at residue 115 as shown below:

–Asn–Leu–Phe–Cys–Ser–Glu–Glu–Met–Pro–Ser–Ser–Asp–Asp–Glu–

Functional studies of TAg proteins bearingmutationswithin this segment (amino acid positions 106 to 114, inclusive) demonstrate that certain deleterious mutations abolishmalignant transformingactivity.[10]For example, mutation of the invariantGluat position 107 toLys-107 completely abolishes transforming activity.[10]Deleterious mutations within this segment (amino acid positions 105 to 114, inclusive) also impair binding of the mutant TAg protein species topRb,[1]implying a correlation between transforming activity and the ability of TAg to bind pRb.[1]A detailed computerizedbioinformaticsanalysis,[9]as well as anx-ray crystallographystudy,[11]have demonstrated thebiophysicalbasis for the interaction between this region of TAg and pRb. TAg residues 103 to 109 form an extended loop structure that binds tightly in a surface groove of pRb.[11]In the crystal structure,Leu-103 is positioned so that it makesvan der Waalscontacts with the hydrophobic side chains ofVal-714 andLeu-769 in pRb.[11]A number ofhydrogen bondsalso stabilize the TAg–pRb complex.[11]For example, the side chain of Glu-107 forms hydrogen bonds by accepting hydrogens from the main chainamidegroups ofPhe-721 andLys-722 in pRb.[11]The mutation ofGlu-107 toLys-107 is expected to result in loss of these hydrogen bonds.[11]Furthermore, the side chain ofLys-107 would likely have energetically unfavorable interactions with the amide ofPhe-721 orLys-722,[11]destabilizing the complex.

Strong experimental evidence confirms that positively charged amino acids (LysorArg) significantly weaken the binding interaction with pRB when positioned in the vicinity of theLeu– x –Cys– x –Glusequence.[12]This is likely due to the fact that the binding surface on pRb features six lysine residues, which will tend to repel positive residues within or flanking theLeu– x –Cys– x –Glusequence.[12]

Of note, the highest-risk oncogenichuman papillomavirus(HPV) strains (16, 18, 31, 45) encode E7 proteins featuring high-affinity pRb-binding domains which match the diagnostic pattern given above.[9]

References

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  1. ^abcDeCaprio JA, Ludlow JW, Figge J, Shew JY, Huang CM, Lee WH, Marsillo E, Paucha E, Livingston DM (15 July 1988)."SV40 large tumor antigen forms a specific complex with the product of the retinoblastoma susceptibility gene".Cell.54(2): 275–83.doi:10.1016/0092-8674(88)90559-4.PMID2839300.S2CID37600468.
  2. ^Ahuja D, Sáenz-Robles MT,Pipas JM(2005)."SV40 large T antigen targets multiple cellular pathways to elicit cellular transformation".Oncogene.24(52): 7729–45.doi:10.1038/sj.onc.1209046.PMID16299533.
  3. ^Ali SH, DeCaprio JA (2001). "Cellular transformation by SV40 large T antigen: interaction with host proteins".Semin Cancer Biol11 (1): 15–23.Archived2004-01-19 at theWayback Machine
  4. ^Sweet BH, Hilleman MR (November 1960). "The vacuolating virus, S.V. 40".Proc. Soc. Exp. Biol. Med.105(2): 420–427.doi:10.3181/00379727-105-26128.PMID13774265.S2CID38744505.
  5. ^P03070;InterPro view for P03070.
  6. ^abDingwall C, Laskey RA (December 1991). "Nuclear targeting sequences – a consensus?".Trends Biochem. Sci.16(12): 478–81.doi:10.1016/0968-0004(91)90184-W.PMID1664152.
  7. ^Fontes MR, Teh T, Kobe B (April 2000). "Structural basis of recognition of monopartite and bipartite nuclear localization sequences by mammalian importin-alpha".J. Mol. Biol.297(5): 1183–94.doi:10.1006/jmbi.2000.3642.PMID10764582.
  8. ^Figge J, Smith TF (14 July 1988)."Cell division sequence motif".Nature.334(6178): 109.doi:10.1038/334109a0.PMID3290690.
  9. ^abcdefghFigge J, Breese K, Vajda S, Zhu QL, Eisele L, Andersen TT, MacColl R, Friedrich T, Smith TF (February 1993)."The binding domain structure of retinoblastoma-binding proteins".Protein Science.2(2): 155–64.doi:10.1002/pro.5560020204.PMC2142352.PMID8382993.
  10. ^abChen S, Paucha E (July 1990)."Identification of a region of simian virus 40 large T antigen required for cell transformation".Journal of Virology.64(7): 3350–7.doi:10.1128/JVI.64.7.3350-3357.1990.PMC249578.PMID2161944.
  11. ^abcdefgKim HY, Ahn BY, Cho Y (15 January 2001)."Structural basis for the inactivation of retinoblastoma tumor suppressor by SV40 large T antigen".The EMBO Journal.20(1–2): 295–304.doi:10.1093/emboj/20.1.295.PMC140208.PMID11226179.
  12. ^abSingh M, Krajewski M, Mikolajka A, Holak TA (11 November 2005)."Molecular determinants for the complex formation between the retinoblastoma protein and LXCXE sequences".The Journal of Biological Chemistry.280(45): 37868–76.doi:10.1074/jbc.M504877200.PMID16118215.
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