title: High-risk (HPV16) human papillomavirus E7 oncoprotein is highly stable and extended, with conformational transitions that could explain its multiple cellular binding partners.
abstract: High-risk papillomaviruses are known to exert their transforming activity mainly through E7, one of their two oncoproteins. Despite its relevance, no structural information has been obtained that could explain the apparent broad binding specificity of E7. Recombinant E7 from HPV-16 purified to near homogeneity showed two species in gel filtration chromatography, one of these corresponding to a dimer with a molecular weight of 22 kDa, determined by multiangle light scattering. The E7 dimer was isolated for characterization and was shown to undergo a substantial conformational transition when changing from pH 7.0 to 5.0, with an increase in helical structure and increased solvent accessibility to hydrophobic surfaces. The protein was resistant to thermal denaturation even in the presence of SDS, and we show that persistent residual structure in the monomer is responsible for its reported anomalous electrophoretic behavior. The dimer also displays a nonglobular hydrodynamic volume based on gel filtration experiments and becomes more globular in the presence of 0.3 M guanidinium chloride, with hydrophobic surfaces becoming accessible to the solvent, as indicated by the large increase in ANS binding. At low protein concentration, dissociation of the globular E7 dimer was observed, preceding the cooperative unfolding of the structured and extended monomer. Although E7 bears properties that resemble natively unfolded polypeptides, its far-UV circular dichroism spectrum, cooperative unfolding, and exposure of ANS binding sites support a folded and extended, as opposed to disordered and fluctuating, conformation. The large increase in solvent accessibility to hydrophobic surfaces upon small pH decrease within physiological range and in mild denaturant concentrations suggests conformational properties that could have evolved to enable protein-protein recognition of the large number of cellular binding partners reported.
authors.affiliation: Instituto Leloir, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Patricias Argentinas 435, (1405) Buenos Aires, Argentina.
authors.forename: Leonardo G
authors.initials: LG
authors.lastname: Alonso
authors.forename: Maria M
authors.initials: MM
authors.lastname: García-Alai
authors.forename: Alejandro D
authors.initials: AD
authors.lastname: Nadra
authors.forename: Alicia N
authors.initials: AN
authors.lastname: Lapeña
authors.forename: Fabio L
authors.initials: FL
authors.lastname: Almeida
authors.forename: Peter
authors.initials: P
authors.lastname: Gualfetti
authors.forename: Gonzalo De
authors.initials: GD
authors.lastname: Prat-Gay
mesh_terms: D000817:Anilino Naphthalenesulfonates / Q000737:chemistry; D002472:Cell Transformation, Viral; D002942:Circular Dichroism; D019281:Dimerization; D004591:Electrophoresis, Polyacrylamide Gel; D019791:Guanidine / Q000737:chemistry; D006358:Hot Temperature; D006801:Humans; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D009856:Oncogene Proteins, Viral / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*; D027383:Papillomaviridae / Q000737:chemistry* / Q000472:pathogenicity; D050725:Papillomavirus E7 Proteins; D011485:Protein Binding; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding; D012307:Risk Factors; D012967:Sodium Dodecyl Sulfate / Q000737:chemistry; D012997:Solvents
chemical_list: D000817:Anilino Naphthalenesulfonates; D009856:Oncogene Proteins, Viral; D050725:Papillomavirus E7 Proteins; D012997:Solvents; C059731:oncogene protein E7, Human papillomavirus type 16; D012967:Sodium Dodecyl Sulfate; C027132:1-anilino-8-naphthalenesulfonate; D019791:Guanidine