¹H and ¹⁵N resonance assignment, secondary structure and dynamic behaviour of the C-terminal domain of human papillomavirus oncoprotein E6

Abstract

E6 is a viral oncoprotein implicated in cervical cancers, produced by human papillomaviruses (HPVs). E6 contains two putative zinc-binding domains of about 75 residues each. The difficulty in producing recombinant E6 has long hindered the obtention of structural data. Recently, we described the expression and purification of E6-C 4C/4S, a stable, folded mutant of the C-terminal domain of HPV16 E6. Here, we have produced ¹⁵N-labelled samples of E6-C 4C/4S for structural studies by NMR. We have assigned most ¹H and ¹⁵N resonances and identified the elements of secondary structure of the domain. The domain displays an original α/β topology with roughly equal proportions of α-helix and β-sheet. The PDZ-binding region of E6, located at the extreme C-terminus of the domain, is in a random conformation. Mass spectrometry demonstrated the presence of one zinc ion per protein molecule. Kinetics of replacement of zinc by cadmium followed by ¹H,¹⁵N-HSQC experiments revealed specific frequency changes for the zinc-binding cysteines and their immediate neighbours. NMR spectra were affected by severe line-broadening effects which seriously hindered the assignment work. Investigation of these effects by ¹⁵N relaxation experiments showed that they are due to heterogeneous dynamic behaviour with μs–ms time scale motions occurring in localised regions of the monomeric domain.