Picornavirus Molecular Biology

The picornavirus family of viruses includes a number of important pathogens of animals and humans, for example foot-and-mouth disease virus, poliovirus and human rhinovirus (the common cold). These viruses are small and simple, comprising a single-stranded RNA genome within a non-enveloped protein capsid.

We are interested in the molecular mechanisms involved in picornavirus cell entry, replication, capsid assembly and genome packaging, and the application of this knowledge towards improved control of foot-and-mouth disease. We use a variety of approaches in our research such as: cell culture, fluorescent microscopy, molecular biology, recombinant proteins, model membranes, electron microscopy and X-ray crystallography. Much of this research is in collaboration with universities such as Leeds, Oxford, Imperial and Harvard.

Foot-and-mouth disease virus (FMDV) genome release

FMDV enters cells by endocytosis and becomes exposed to low pH in acidified endosomes. This triggers the virus capsid to dissociate into subunits and results in the RNA genome being transported, by an unknown mechanism, across the endosomal membrane into the cytoplasm for infection to begin. We have recently investigated these events for the very closely related picornavirus equine rhinitis A virus (ERAV). In these studies we identified the low pH-induced formation of a transient empty particle from which the genome had been released (i.e. before capsid dissociation) and determined novel structures of the native virus and empty particles by X-ray crystallography (click here for the paper). These findings challenge the existing notion of capsid dissociation being the mechanism for genome release and suggest instead that membrane penetration and genome delivery involve an intact (but altered) particle. We are now beginning to identify and characterize similar particles for FMDV.

Picornavirus-membrane interactions

We use liposomes (unilamellar vesicles) as model membranes to study the membrane interactions of picornaviruses in defined cell-free conditions. Receptor-decorated liposomes bind poliovirus, triggering alterations in the capsid structure that enable the altered particles to interact directly with the membrane (click here for the paper). By triggering these events in the presence of dye-filled liposomes, the permeability induced by these membrane interactions can be characterised by the release of dye. VP4 is a small internal capsid protein which is released from some picornaviruses to interact with membranes during cell entry. We have shown that recombinant forms of this protein can interact with and permeabilise liposome membranes (click here for the paper), suggesting a role for VP4 in virus entry. We are using these approaches to further investigate the membrane interactions of infectious virus particles and the role of specific capsid components.

Assembly of FMDV

The picornavirus capsid precursor protein (P1 polyprotein) is processed by the viral protease, after which it self-assembles into pentameric structures. Twelve pentamers then form complete capsids. Empty particles are produced in recombinant systems; virus particles (containing genome) and some empty particles are produced in infected cells. Little else is known about how the capsid assembles or how the RNA genome is packaged. We have demonstrated the processing and self-assembly of FMDV pentamers in an entirely recombinant and cell-free system (click here for the paper) and are now using this approach in combination with studies in infected cells to investigate the mechanisms of pentamer and capsid assembly.

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