Structural Insights in Multifunctional Papillomavirus Oncoproteins

Abstract

Since their discovery in the mid-eighties, the main papillomavirus oncoproteins E6 and E7 have been recalcitrant to high-resolution structure analysis. However, in the last decade a wealth of three-dimensional information has been gained on both proteins whether free or complexed to host target proteins. Here, we first summarize the diverse activities of these small multifunctional oncoproteins. Next, we review the available structural data and the new insights they provide about the evolution of E6 and E7, their multiple interactions and their functional variability across human papillomavirus (HPV) species.

Keywords: NMR; X-ray; oncoproteins; papillomaviruses; structure; virus-host interactomics.

Figure 1

Schematics of E6 and E7 sequences across distinct vertebrates: (A) mammals; (B) birds; (C) turtles. Folded zinc-binding domains and unfolded regions are represented as rectangles and extended strings, respectively. Approximate amino-acid numbering is indicated below the schemes.

Figure 2

Architecture of mammalian human papillomavirus (HPV) E6. (A) Secondary structure elements of mammalian HPV16 E6. E6N is shaded in blue, E6C is shaded in gray; (B) Superimposition of HPV16 E6N domain solved by NMR [91] and HPV16 E6N domain from the crystal structure of the E6/E6AP LxxLL complex [75]; (C) Superimposition of HPV16 E6C domain from the crystal structure of the E6/E6AP LxxLL complex [75], HPV51 E6C domain solved by NMR [101] and bovine papillomavirus 1 (BPV1) E6C domain from the crystal structure of the BPV1 E6/paxillin LxxLL complex [75]; (D) Superimposition of HPV16 E6N and E6C domains from the crystal structure of the HPV16 E6/E6AP LxxLL complex [75]; (E) Structure of LxxLL-bound mammalian HPV16 E6 [75] (LxxLL motif not shown for clarity). Zinc-coordinating cysteines are highlighted in red. Secondary elements are numbered as in [75]. Spheres are Zinc (II) atoms. All structural views in this article were prepared using Pymol (http://www.pymol.org/).

Figure 3

Structure of mammalian papillomavirus (PV) E7 ZBD and comparison to mammalian PV E6N and E6C. (A) Schematics of mammalian HPV E7 sequence. Mammalian PV E7 proteins contain an intrinsically unfolded N-terminus bearing a conserved LxCxE sequence (as indicated on the sheme) among other putative interaction motifs, followed by a folded zinc-binding domain (E7-ZBD) shaded in green, with four conserved cysteine (C) residues as indicated; (B) Solution structure of HPV45 E7 ZBD homodimer [76]. A comparable structure was also obtained by crystallography for the E7 ZBD of HPV1 E7 [78]; (C) Elements of the treble clef motif of E7 ZBD (in red) in context of the E7 ZBD monomer; (D) Elements of the treble clef motif of E7 ZBD (in red) in context of the E7 ZBD dimer; (E) Elements of the treble clef motif of E6N (in red); (F) Elements of the treble clef motif of E6C (in red); (G) Superimposition of HPV1 E7-ZBD and HPV16 E6N, enlighting common features of their treble clef motifs, indicated in orange and red color, respectively. Spheres are Zinc (II) atoms.

Figure 4

Main structural data on HPV16 E6 and E7 bound to cellular targets. (A) Crystal structure of full-length HPV16 E6 bound to the LxxLL motif of E6AP [75] LxxLL binding residues are highlighted in red; (B) Crystal structure of HPV16 E6 bound to the LxxLL motif of E6AP and the core domain of p53 [98]; (C) Solution structure of MAGI-1 (membrane associated guanylate kinase inverted 1) PDZ 2/6 bound to the C-terminal PDZ-Binding Motif (PBM) of HPV16 E6 [115]. LxxLL binding residues are highlighted in red; (D) Crystal structure of p107 pocket domain (composed of A-box and B-box) bound to the LxCxE motif of HPV16 E7 [116]. A comparable crystal structure was previously obtained for pRb pocket domain bound to the LxCxE motif of HPV16 E7 [117].