Dissecting Quasi-Equivalence in Nonenveloped Viruses: Membrane Disruption Is Promoted by Lytic Peptides Released from Subunit

Nonenveloped viruses often invade membranes by exposing hydrophobic or amphipathic peptides generated by a proteolytic maturation step that leaves a lytic peptide noncovalently associated with the viral capsid. Since multiple copies of the same pro-tein formmany nonenveloped virus capsids, it is unclear if lytic peptides derived from subunits occupying different positions in a quasi-equivalent icosahedral capsid play different roles in host infection. We addressed this question withNudaurelia capensis omega virus (NV), an insect RNA virus with an icosahedral capsid formed by protein , which undergoes autocleavage during maturation, producing the lytic peptide. NV is a unique model because autocatalysis can be precisely initiated in vitro and is sufficiently slow to correlate lytic activity with peptide production. Using liposome-based assays, we observed that autocataly-sis is essential for the potent membrane disruption caused by NV.We observed that lytic activity is acquired rapidly during the maturation program, reaching 100 % activity with less than 50 % of the subunits cleaved. Previous time-resolved structural stud-ies of partially mature NV particles showed that, during this time frame, peptides derived from the pentamer subunits are produced and are organized in a vertical helical bundle that is projected toward the particle surface, while identical polypeptides in quasi-equivalent subunits are produced later or are in positions inappropriate for release. Our functional data provide experi-mental support for the hypothesis that pentamers containing a central helical bundle, observed in different nonenveloped virus families, are a specialized lytic motif. Animal virus infection generally involves an interaction of theparticle with a cell surface receptor, followed by any of a num