Poliovirus infection requires that the particle undergo a series of conformational transitions that lead to cell entry and genome release. In an effort to understand the conformational changes associated with the release of the RNA genome, we have used cryo-electron microscopy to characterize the structure of the 80S "empty" particles of poliovirus that are thought to represent the final product of the cell entry pathway. Using two-dimensional classification methods, we show that preparations of 80S particles contain at least two structures, which might represent snapshots from a continuous series of conformers. Using three-dimensional reconstruction methods, we have solved the structure of two distinct forms at subnanometric resolution, and we have built and refined pseudoatomic models into the reconstructions. The reconstructions and the derived models demonstrate that the two structural forms are both slightly expanded, resulting in partial disruption of interprotomer interfaces near their particle 2-fold axes, which may represent the site where RNA is released. The models demonstrate that each of the two 80S structures has undergone a unique set of movements of the capsid proteins, associated with rearrangement of flexible loops and amino-terminal extensions that participate in contacts between protomers, between pentamers, and with the viral RNA.Like all members of the picornavirus family, poliovirus has an icosahedral capsid (300 Å in diameter) consisting of 60 copies of each of four coat proteins, VP1, VP2, VP3, and myrVP4, which encapsidates a plus-sense single-stranded RNA genome (Fig. 1A). The poliovirus capsid proteins are responsible for several functions along the infection pathway, including programmed assembly, encapsidation of the viral genome, protection of the genome, host cell recognition, cell entry (involving both membrane binding and penetration), and release and delivery of the viral genome. This variation in function requires both individual capsid proteins and the capsid as a whole to undergo a series of triggered dynamic structural changes.Several structurally unique assembly and disassembly intermediates have been described for poliovirus, which can be distinguished by sedimentation properties on a sucrose gradient and by antigenicity (16,35,42,48). During assembly, all four capsid protein sequences are translated together as a single polypeptide that is cotranslationally myristoylated. This precursor form of the protomer, called myrP1, sediments at 7S. Once the protomer has been cleaved by virally encoded protease to yield myrVP0, VP3, and VP1, subsequent rearrangements of the freed amino and carboxyl termini allow protomers to assemble into pentamers that sediment at 14S. In turn, pentamers can assemble into an empty-capsid intermediate that sediments at 73S, which contains 60 copies of myrVP0, VP3, and VP1. Encapsidation of the viral genome is associated with the autocatalytic cleavage of myrVP0 to yield myrVP4 and VP2. This final cleavage event results in a significant increase in...

After recognizing and binding to its host cell, poliovirus (like other nonenveloped viruses) faces the challenge of translocating its genome across a cellular membrane and into the cytoplasm. To avoid entanglement with the capsid, the RNA must exit via a single site on the virion surface. However, the mechanism by which a single site is selected (from among 60 equivalents) is unknown; and until now, even its location on the virion surface has been controversial. To help to elucidate the mechanism of infection, we have used single-particle cryoelectron microscopy and tomography to reconstruct conformationally altered intermediates that are formed by the poliovirion at various stages of the poliovirus infection process. Recently, we reported icosahedrally symmetric structures for two forms of the end-state 80S empty capsid particle. Surprisingly, RNA was frequently visible near the capsid; and in a subset of the virions, RNA was seen on both the inside and outside of the capsid, caught in the act of exiting. To visualize RNA exiting, we have now determined asymmetric reconstructions from that subset, using both single-particle cryo-electron microscopy and cryo-electron tomographic methods, producing independent reconstructions at ϳ50-Å resolution. Contrary to predictions in the literature, the footprint of RNA on the capsid surface is located close to a viral 2-fold axis, covering a slot-shaped area of reduced density that is present in both of the symmetrized 80S reconstructions and which extends by about 20 Å away from the 2-fold axis toward each neighboring 5-fold axis.

Adeno-associated virus serotype 2 (AAV2) uses heparan sulfate proteoglycan as a cell surface-attachment receptor. In this study the structures of AAV2 alone and complexed with heparin were determined to ~18 Å resolution using cryo-electron microscopy and three-dimensional image reconstruction. A difference map showed positive density, modeled as heparin, close to the icosahedral twofold axes and between the protrusions that surround the threefold axes of the capsid. Regions of the model near the threefold place the receptor in close proximity to basic residues previously identified as part of the heparin binding site. The region of the model near the twofold axes identifies a second contact site, not previously characterized but which is also possibly configured by heparin binding. The difference map also revealed two significant conformational changes: I) at the tops of the threefold protrusions, which have become flattened in the complex, and II) at the fivefold axes where the top of the channel is widened possibly in response to movement of the HI loops in the capsid proteins. Ordered density in the interior of the capsid in the AAV2-heparin complex was interpreted as nucleic acid, consistent with the presence of non-viral DNA in the expressed capsids.

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Adeno-associated virus (AAV) is a member of the Parvoviridae, belonging to the Dependovirus genus. Currently, several distinct isolates of AAV are in development for use in human gene therapy applications due to their ability to transduce different target cells. The need to manipulate AAV capsids for specific tissue delivery has generated interest in understanding their capsid structures. The structure of AAV type 4 (AAV4), one of the most antigenically distinct serotypes, was determined to 13-Å resolution by cryo-electron microscopy and image reconstruction. A pseudoatomic model was built for the AAV4 capsid by use of a structure-based sequence alignment of its major capsid protein, VP3, with that of AAV2, to which AAV4 is 58% identical and constrained by its reconstructed density envelope. The model showed variations in the surface loops that may account for the differences in receptor binding and antigenicity between AAV2 and AAV4. The AAV4 capsid surface topology also shows an unpredicted structural similarity to that of Aleutian mink disease virus and human parvovirus B19, autonomous members of the genus, despite limited sequence homology.

c During infection, the binding of poliovirus to its cell surface receptor at 37°C triggers an expansion of the virus in which internal polypeptides that bind to membranes are externalized. Subsequently, in a poorly understood process, the viral RNA genome is transferred directly across an endosomal membrane, and into the host cell cytoplasm, to initiate infection. Here, cryoelectron tomography demonstrates the results of 37°C warming of a poliovirus-receptor-liposome model complex that was produced using Ni-nitrilotriacetic acid lipids and His-tagged receptor ectodomains. In total, 651 subtomographic volumes were aligned, classified, and averaged to obtain detailed pictures, showing both the conversion of virus into its expanded form and the passage of RNA into intact liposomes. Unexpectedly, the virus and membrane surfaces were located ϳ50 Å apart, with the 5-fold axis tilted away from the perpendicular, and the solvent spaces between them were spanned by either one or two long "umbilical" density features that lie at an angle to the virus and membrane. The thinner connector, which sometimes appears alone, is 28 to 30 Å in diameter and has a footprint on the virus surface located close to either a 5-fold or a 3-fold axis. The broader connector has a footprint near the quasi-3-fold hole that opens upon virus expansion and is hypothesized to include RNA, shielded from enzymatic degradation by polypeptides that include the N-terminal extension of VP1 and capsid protein VP4. The implications of these observations for the mechanism of RNase-protected RNA transfer in picornaviruses are discussed.

Picornaviruses are non-enveloped RNA viruses that enter cells via receptor-mediated endocytosis. Because they lack an envelope, picornaviruses face the challenge of delivering their RNA genomes across the membrane of the endocytic vesicle into the cytoplasm to initiate infection. Currently, the mechanism of genome release and translocation across membranes remains poorly understood. Within the enterovirus genus, poliovirus, rhinovirus 2, and rhinovirus 16 have been proposed to release their genomes across intact endosomal membranes through virally induced pores, whereas one study has proposed that rhinovirus 14 releases its RNA following disruption of endosomal membranes. For the more distantly related aphthovirus genus (e.g. foot-and-mouth disease viruses and equine rhinitis A virus) acidification of endosomes results in the disassembly of the virion into pentamers and in the release of the viral RNA into the lumen of the endosome, but no details have been elucidated as how the RNA crosses the vesicle membrane. However, more recent studies suggest aphthovirus RNA is released from intact particles and the dissociation to pentamers may be a late event. In this study we have investigated the RNase A sensitivity of genome translocation of poliovirus using a receptor-decorated-liposome model and the sensitivity of infection of poliovirus and equine-rhinitis A virus to co-internalized RNase A. We show that poliovirus genome translocation is insensitive to RNase A and results in little or no release into the medium in the liposome model. We also show that infectivity is not reduced by co-internalized RNase A for poliovirus and equine rhinitis A virus. Additionally, we show that all poliovirus genomes that are internalized into cells, not just those resulting in infection, are protected from RNase A. These results support a finely coordinated, directional model of viral RNA delivery that involves viral proteins and cellular membranes.

Adeno-associated virus serotype 5 (AAV5) is under development for genetherapy applications for the treatment of cystic fibrosis. To elucidate the structural features of AAV5 that control its enhanced transduction of the apical surface of airway epithelia compared with other AAV serotypes, X-ray crystallographic studies of the viral capsid have been initiated. The production, purification, crystallization and preliminary crystallographic analysis of empty AAV5 viral capsids are reported. The crystals diffract X-rays to beyond 3.2 Å resolution using synchrotron radiation and belong to the orthorhombic space group P2 1 2 1 2 1 , with unit-cell parameters a = 264.7, b = 447.9, c = 629.7 Å . There is one complete T = 1 viral capsid per asymmetric unit. The orientation and position of the viral capsid in the asymmetric unit have been determined by rotation and translation functions, respectively, and the AAV5 structure determination is in progress.