Flock house virus: a model system for understanding non-enveloped virus entry and membrane penetration.

The means by which non-enveloped viruses penetrate cellular membranes during cell entry remain poorly defined. Recent findings indicate that several members of this group share a common mechanism of membrane penetration in which the virus particle undergoes programmed conformational changes, leading to capsid disassembly and release of small membrane-interacting peptides. Flock House Virus (FHV), a member of the nodaviridae family, offers some unique advantages for studying non-enveloped virus entry.

Low endocytic pH and capsid protein autocleavage are critical components of Flock House virus cell entry.

The process by which nonenveloped viruses cross cell membranes during host cell entry remains poorly defined; however, common themes are emerging. Here, we use correlated in vivo and in vitro studies to understand the mechanism of Flock House virus (FHV) entry and membrane penetration. We demonstrate that low endocytic pH is required for FHV infection, that exposure to acidic pH promotes FHV-mediated disruption of model membranes (liposomes), and particles exposed to low pH in vitro exhibit increased hydrophobicity.

Dissecting the functional domains of a nonenveloped virus membrane penetration peptide.

Recent studies have established that several nonenveloped viruses utilize virus-encoded lytic peptides for host membrane disruption. We investigated this mechanism with the "gamma" peptide of the insect virus Flock House virus (FHV). We demonstrate that the C terminus of gamma is essential for membrane disruption in vitro and the rescue of immature virus infectivity in vivo, and the amphipathic N terminus of gamma alone is not sufficient.

Putative autocleavage of reovirus mu1 protein in concert with outer-capsid disassembly and activation for membrane permeabilization.

Capsid proteins of several different families of non-enveloped animal viruses with single-stranded RNA genomes undergo autocatalytic cleavage (autocleavage) as a maturation step in assembly. Similarly, the 76 kDa major outer-capsid protein mu1 of mammalian orthoreoviruses (reoviruses), which are non-enveloped and have double-stranded RNA genomes, undergoes putative autocleavage between residues 42 and 43, yielding N-terminal N-myristoylated fragment mu1N and C-terminal fragment mu1C. Cleavage at this site allows release of mu1N, which is thought to be critical for penetration of the host-cell membrane during cell entry.

Putative autocleavage of outer capsid protein micro1, allowing release of myristoylated peptide micro1N during particle uncoating, is critical for cell entry by reovirus.

Several nonenveloped animal viruses possess an autolytic capsid protein that is cleaved as a maturation step during assembly to yield infectious virions. The 76-kDa major outer capsid protein micro1 of mammalian orthoreoviruses (reoviruses) is also thought to be autocatalytically cleaved, yielding the virion-associated fragments micro1N (4 kDa; myristoylated) and micro1C (72 kDa). In this study, we found that micro1 cleavage to yield micro1N and micro1C was not required for outer capsid assembly but contributed greatly to the infectivity of the assembled particles.

Disulfide bonding among micro 1 trimers in mammalian reovirus outer capsid: a late and reversible step in virion morphogenesis.

We examined how a particular type of intermolecular disulfide (ds) bond is formed in the capsid of a cytoplasmically replicating nonenveloped animal virus despite the normally reducing environment inside cells. The micro 1 protein, a major component of the mammalian reovirus outer capsid, has been implicated in penetration of the cellular membrane barrier during cell entry. A recent crystal structure determination supports past evidence that the basal oligomer of micro 1 is a trimer and that 200 of these trimers surround the core in the fenestrated T=13 outer capsid of virions.