title: Six molecules of SV40 large T antigen assemble in a propeller-shaped particle around a channel.
abstract: The large T antigen of simian virus 40 (SV40) is a multifunctional regulatory protein, responsible for both the control of viral infection and the required alterations of cellular processes. T antigen is the only viral protein required for viral DNA replication. It binds specifically to the viral origin and as a helicase unwinds the SV40 DNA bidirectionally. The functional complex is a double hexameric oligomer. In the absence of DNA, but in the presence of ATP or a non-hydrolyzable analog, T antigen assembles into hexamers, which are active as a helicase when a partially single-stranded (3') entry site exists on the substrate. We have used negative staining electron microscopy, single particle image processing and three-dimensional reconstruction with a new algebraic reconstruction techniques (ART) algorithm to study the structure of these hexameric particles in the presence of different nucleotide cofactors (ATP, ADP, and the non-hydrolyzable analogs ATPgammaS and AMP-PNP). In every case a strong 6-fold structure was found, with the six density maxima arranged in a ring-like particle around a channel, and a well-defined vorticity. Because these structural features have recently been found in other prokaryotic helicases, they seem to be strongly related to the activity of the protein, which suggests a general functional model conserved through evolution.
authors.affiliation: Centro Nacional de Biotecnología (C.S.I.C.), Universidad Autónoma de Madrid, Cantoblanco, Spain.
authors.forename: M C
authors.initials: MC
authors.lastname: San Martín
authors.forename: C
authors.initials: C
authors.lastname: Gruss
authors.forename: J M
authors.initials: JM
authors.lastname: Carazo
mesh_terms: D000244:Adenosine Diphosphate / Q000737:chemistry / Q000378:metabolism; D000266:Adenylyl Imidodiphosphate / Q000737:chemistry / Q000378:metabolism; D000465:Algorithms; D000952:Antigens, Polyomavirus Transforming / Q000737:chemistry* / Q000378:metabolism*; D007091:Image Processing, Computer-Assisted; D008854:Microscopy, Electron / Q000379:methods; D008958:Models, Molecular; D011487:Protein Conformation; D013539:Simian virus 40 / Q000276:immunology*
chemical_list: D000952:Antigens, Polyomavirus Transforming; D000266:Adenylyl Imidodiphosphate; D000244:Adenosine Diphosphate