The Picornaviridae family of small, nonenveloped viruses includes major pathogens of humans and animals. They have positivesense, single-stranded RNA genomes, and the mechanism(s) by which these genomes are introduced into cells to initiate infection remains poorly understood. The structures of presumed uncoating intermediate particles of several picornaviruses show limited expansion and some increased porosity compared to the mature virions. Here, we present the cryo-electron microscopy

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... re of native equine rhinitis A virus (ERAV), together with the structure of a massively expanded ERAV particle, each at ϳ17-Å resolution. The expanded structure has large pores on the particle 3-fold axes and has lost the RNA genome and the capsid protein VP4. The expanded structure thus illustrates both the limits of structural plasticity in such capsids and a plausible route by which genomic RNA might exit. IMPORTANCE Picornaviruses are important animal and human pathogens that protect their genomic RNAs within a protective protein capsid. Upon infection, this genomic RNA must be able to leave the capsid to initiate a new round of infection. We describe here the structure of a unique, massively expanded state of equine rhinitis A virus that provides insight into how this exit might occur.