The HPV16 E7 Viral Oncoprotein Self-Assembles into Defined Spherical Oligomers

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Abstract
Despite the fact that E7 is a major transforming oncoprotein in papillomavirus, its structure and precise molecular mechanism of action remain puzzling to date. E7 proteins share sequence homology and proteasome targeting properties of tumor suppressors with adenovirus E1A and SV40 T antigen, two other paradigmatic oncoproteins from DNA tumor viruses. High-risk HPV16 E7, a nonglobular dimer with some properties of intrinsically disordered proteins, is capable of undergoing pH-dependent conformational transitions that expose hydrophobic surfaces to the solvent. We found that treatment with a chelating agent produced a protein that can readily assemble into homogeneous spherical particles with an average molecular mass of 790 kDa and a diameter of 50 nm, as determined from dynamic light scattering and electron microscopy. The protein undergoes a substantial conformational transition from coil to β-sheet structure, with concomitant consolidation of tertiary structure as judged by circular dichroism and fluorescence. The assembly process is very slow, in agreement with a substantial energy barrier caused by structural rearrangements. The resulting particles are highly stable, cooperatively folded, and capable of binding both Congo Red and thioflavin T, reporters of repetitive β-sheet structures similar to those found in amyloids, although no fibrillar or insoluble material was observed under our experimental conditions.