title: Large T-antigen double hexamers imaged at the simian virus 40 origin of replication.
abstract: The initial step of simian virus 40 (SV40) DNA replication is the binding of the large tumor antigen (T-Ag) to the SV40 core origin. In the presence of Mg(2+) and ATP, T-Ag forms a double-hexamer complex covering the complete core origin. By using electron microscopy and negative staining, we visualized for the first time T-Ag double hexamers bound to the SV40 origin. Image processing of side views of these nucleoprotein complexes revealed bilobed particles 24 nm long and 8 to 12 nm wide, which indicates that the two T-Ag hexamers are oriented head to head. Taking into account all of the biochemical data known on the T-Ag-DNA interactions at the replication origin, we present a model in which the DNA passes through the inner channel of both hexamers. In addition, we describe a previously undetected structural domain of the T-Ag hexamer and thereby amend the previously published dimensions of the T-Ag hexamer. This domain we have determined to be the DNA-binding domain of T-Ag.
authors.affiliation: Centro Nacional de Biotecnología (CSIC), Campus de Cantoblanco, 28049 Madrid, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Valle
authors.forename: C
authors.initials: C
authors.lastname: Gruss
authors.forename: L
authors.initials: L
authors.lastname: Halmer
authors.forename: J M
authors.initials: JM
authors.lastname: Carazo
authors.forename: L E
authors.initials: LE
authors.lastname: Donate
mesh_terms: D000957:Antigens, Viral, Tumor / Q000737:chemistry / Q000235:genetics*; D001665:Binding Sites / Q000235:genetics; D004261:DNA Replication / Q000235:genetics; D004279:DNA, Viral / Q000737:chemistry / Q000235:genetics*; D011485:Protein Binding; D018741:Replication Origin / Q000235:genetics*; D013539:Simian virus 40 / Q000235:genetics*
chemical_list: D000957:Antigens, Viral, Tumor; D004279:DNA, Viral