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Contents 1 SV40 Large T Antigen 1.1 Introduction 1.2 Structure and Function 1.2.1 Helicase 1.2.2 References

SV40 Large T Antigen

Introduction

The SV40 large tumor antigen is a multifunctional regulatory protein encoded by Simian Virus 40. It is classified under the AAA+ family of helicases ^[1]. Noteworthy for its versatility, the protein is responsible for initiation of viral DNA replication, regulation of viral transcription and transformation of the host cell to promote viral infectivity. Large T-antigen is an early gene product of SV40 and is produced via differential mRNA splicing.

Structure and Function

T antigen is a 708-amino acid protein consisting of three major domains: an N-terminal J domain, a central origin-binding domain, and a C-terminal helicase domain ^[2].

spinqualitylabelsall models The origin binding domain of SV40 large T antigen Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image The Origin Binding Domain The origin binding domain monomer consists of five anti-parallel beta sheets flanked on either side by a pair of alpha helices. The monomers assemble into a hexameric left-handed spiral, whose pitch complements the turn of DNA. Side-side interaction is necessary for hexamerization, in which residues are crucial. Residues along the are necessary in the assembly of a double hexamer. A central pore 40 Angstroms wide is formed, large enough for dsDNA, carrying positive charge. A monomer is able to bind along a series of GAGGC pentanucleotides P1 through P4 at the origin, collectively known as Site II. implicated in DNA binding are ; ; as well as [1]. The structural fold adopted by the fully assembled double hexamer is similarly conserved across a number of origin-binding proteins in different viruses, despite varying protein sequences, suggesting sequence-specificity. The Asn and Arg in the A1 motif primarily make up base-specific interactions with the DNA, whereas residues from the B2 loop interact mainly with the phosphate backbone. These specific interactions bury a large surface area of the protein and give rise to a 60nM Kd.[3]. Binding of large T antigen at the origin of replication allows replicative machinery to unwind and synthesize new DNA. Residues involved in DNA binding also bind ssDNA-binding protein human RPA[4]. T antigen also acts as a repressor of early gene transcription. When increased amounts of T antigen are present, it binds DNA and blocks the overlapping promoter sequence, thus behaving as its own regulator.

spinqualitylabelsall models PDB ID 1svm Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image
1svm, resolution 1.94Å ()
Ligands:	, ,
Related:	1svl, 1svo
Structural annotation: Resources: CATH : 1Svma03, 1Svma01, 1Svmb01, 1Svmb03, 1Svmc01, 1Svmc03, 1Svmd01, 1Svmd03, 1Svme01, 1Svme03, 1Svmf01, 1Svmf03 InterPro : Ipr014015, Ipr010932, Ipr003133, Ipr001623 Pfam : PF06431 UniProt : P03070
Functional annotation: Cellular component: nucleus Biological process: DNA replication Molecular function: ATP binding heat shock protein binding metal ion binding nucleotide binding zinc ion binding DNA binding DNA replication origin binding protein binding
Evolutionary conservation: Toggle Conservation Colors: Rows = identical sequences: Further Information: Caveats, Explanation, Complete Results at ConSurfDB
Resources:	FirstGlance, OCA, RCSB, PDBsum
Coordinates:	save as pdb, mmCIF, xml

Helicase

The consists of a AAA+ domain. Each monomer binds and hydrolyzes an ATP at this region in the presence of magnesium ion. Together, the monomers drive an overall conformational change in the hexamer. The helicase monomer can exist in one of three states: ATP-bound, ADP-bound and Nt-free. The transitions in conformation between these states enable the unwinding of viral dsDNA. There are both cis- and trans-monomer interactions involving ATP.

Lys 432, Thr 433 and Thr 434 of the P loop interact closely with the triphosphate groups, and Asp 474 and Asn 529 form H bonds with ATP on the same residue. Among these, Ilu 428, Thr 433 and Asp 474 adopt different conformations in the ADP-bound state. In the Nt-free state, Ilu 428 and Thr 434 are turned even further and sterically disallow the presence of ADP.

near the ATP is present for stability and nucleophilic attack during hydrolysis.

, of which Lys 418 and Lys 419 stabilize ATP hydrolysis, interact with ATP on adjacent monomers, important in producing conformational changes. These rearrangements affect the position of a positively-charged structure that protrudes into the central channel. Trans-residues Arg 498, Asp 499 and Asp 502 are located at the base of the hairpin, lending a lever-like functionality. The motion of the β hairpin unwinds the DNA through the central channel ^[5].

spinqualitylabelsall models Large T antigen complexed with retinoblastoma pocket Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image J domain and LxCxE motif In the transformation of the host cell, retinoblastoma, a tumor suppressor, is inactivated via both the N-terminal J domain and the LxCxE motif [6]. The J domain is a molecular chaperone domain [7]. The N-terminal region (residues 7-117), shown to the right, consists of four alpha helices and an extended loop.

References