Large T Antigen: Difference between revisions

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 <ref name="B">PMID:15454080</ref>.

===== 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 [http://www.pdb.org/pdb/explore/jmol.do?structureId=2FUF&bionumber=1 hexameric left-handed spiral], whose pitch complements the turn of DNA. Side-side interaction is necessary for hexamerization, in which residues <scene name='User:Udayan_Shevade/Sandbox1/Obd_183_185/1'>Phe183 and Ser185</scene> are crucial. Residues along the <scene name='User:Udayan_Shevade/Sandbox1/Obd_double_hexamer_residues/1'>B3 motif</scene> are necessary in the assembly of a double hexamer. A central pore is formed, large enough for dsDNA, carrying positive charge. A monomer along this pore is able to bind along a series of GAGGC pentanucleotides P1 through P4 at the origin, collectively known as Site II. <scene name='User:Udayan_Shevade/Sandbox1/Obd_residues/1'>Residues</scene> implicated in DNA binding are <scene name='User:Udayan_Shevade/Sandbox1/Obd_153_154_155/5'>Asn 153, Arg154, Thr155 from the A1 motif</scene>; <scene name='User:Udayan_Shevade/Sandbox1/Obd_203_204/1'>His203, Arg204 from the B2 motif</scene>; as well as <scene name='User:Udayan_Shevade/Sandbox1/Obd_201_202/1'>His201 andArg 202</scene><ref name="A"/>. 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 K_d.<ref name="C">PMID:1779811</ref>.