Structural Analysis Reveals an Amyloid Form of the Human Papillomavirus Type 16 E1 ∧ E4 Protein and Provides a Molecular Basis for Its Accumulation

Abstract

The abundant human papillomavirus (HPV) type 16 E4 protein exists as two distinct structural forms in differentiating epithelial cells. Monomeric full-length 16E1 ^∧ E4 contains a limited tertiary fold constrained by the N and C termini. N-terminal deletions facilitate the assembly of E1 ^∧ E4 into amyloid-like fibrils, which bind to thioflavin T. The C-terminal region is highly amyloidogenic, and its deletion abolishes amyloid staining and prevents E1 ^∧ E4 accumulation. Amyloid-imaging probes can detect 16E1 ^∧ E4 in biopsy material, as well as 18E1 ^∧ E4 and 33E1 ^∧ E4 in monolayer cells, indicating structural conservation. Our results suggest a role for fibril formation in facilitating the accumulation of E1 ^∧ E4 during HPV infection.