A Membrane-Destabilizing Peptide in Capsid Protein L2 Is Required for Egress of Papillomavirus Genomes from Endosomes

Abstract

Papillomaviruses are internalized via clathrin-dependent endocytosis. However, the mechanism by which viral genomes pass endosomal membranes has not been elucidated. In this report we show that the minor capsid protein L2 is required for egress of viral genomes from endosomes but not for initial uptake and uncoating and that a 23-amino-acid peptide at the C terminus of L2 is necessary for this function. Pseudogenomes encapsidated by L1 and L2 lacking this peptide accumulated in vesicular compartments similar to that observed with L1-only viral particles, and these mutant pseudoviruses were noninfectious. This L2 peptide displayed strong membrane-disrupting activity, induced cytolysis of bacteria and eukaryotic cells in a pHdependent manner, and permeabilized cells after exogenous addition. Fusions between green fluorescent protein and the L2 peptide integrated into cellular membranes like the wild type but not like C-terminal mutants of L2. Our data indicate that the L2 C terminus facilitates escape of viral genomes from the endocytic compartment and that this feature is conserved among papillomaviruses. Furthermore, the characteristic of this peptide differs from the classical virus-encoded membrane-penetrating peptides.

Cleavage of the papillomavirus minor capsid protein, L2, at a furin consensus site is necessary for infection

“…Virion binding leads to a conformational change in the capsid that in turn permits furin to cleave the N terminus of L2, which may occur at the cell surface or within an early endosomal compartment. The capsid undergoes uncoating in a late endosomal compartment, leading to furin-cleaved L2 and the associated genome to escape from the endosome into the cytoplasm via a mechanism that also involves the C terminus of L2 (6). Furin inhibition by dec.-RVKR-cmk prevents the escape of L2 and the genome from the endosome but does not appear to interfere with virion trafficking or uncoating.…”

“…The localization of the genome to the transcriptionally active nuclear domain 10, which is critical to the efficient establishment of infection, depends on L2. It has also been demonstrated that a C-terminal region of L2 mediates endosomal escape after viral uncoating (6). Additionally, a recent report showed that L2 interacts with syntaxin 18 during entry and possibly uses this resident endoplasmic reticulum protein as a tether for transport toward the nucleus (7).…”

Human α-defensins block papillomavirus infection

“…Both viruses have recently been shown to carry membrane-destabilizing domains within their capsid structural proteins (67,69). Because defensins are known to interact with membranes, it is possible to imagine that they act against papillomaviruses and adenoviruses by interacting with cellular membranes in a manner that blocks the effects of membrane-destabilizing viral peptides.…”

Biology of Human Papillomavirus Infections in Head and Neck Carcinogenesis

“…L2 is required for infectivity and required for egress of viral genomes from endosomes, while L1 does not appear to exit the endosomal compartment [12,13]. Following endosomal escape, L2 accompanies the viral DNA to the nucleus.…”