FIG. 1.

Models of viral infection and release. (A) Membrane penetration and release of enveloped and nonenveloped viruses. (Top) Fusion of the viral membrane with the host cell membrane is used to overcome the lipid bilayer for enveloped virus infection. Viral replication is facilitated by host cell factors. Enveloped viruses bud from infected cells and are released by membrane fission. (Bottom) Nonenveloped viruses induce host cell lysis for efficient release of particles. (B) Mechanisms of enveloped and nonenveloped viral penetration. (Left) Internalization of influenza virus and trafficking to acidic endosomes leads to activation of the viroporin M2 (green) and proton influx into the virus interior. This is thought to induce structural rearrangements of the virus particle. The penetration of influenza virus is mediated by envelope glycoproteins for delivery of the activated viral particle. (Right) Protease activation or receptor binding induces the release of capsid viroporins from nonenveloped viruses to induce pore formation or membrane lysis for penetration.