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Ubiquitous and abundant in ecosystems and microbiomes, gokushoviruses constitute a Microviridae subfamily, distantly related to bacteriophages ΦX174, α3, and G4. A high-resolution cryo-EM structure of gokushovirus ΦEC6098 was determined, and the atomic model was built de novo. Although gokushoviruses lack external scaffolding and spike proteins, which extensively interact with the ΦX174 capsid protein, the core of the ΦEC6098 coat protein (VP1) displayed a similar structure. There are, however, key differences. At each ΦEC6098 icosahedral 3-fold axis, a long insertion loop formed mushroom-like protrusions, which have been noted in lower-resolution gokushovirus structures. Hydrophobic interfaces at the bottom of these protrusions may confer stability to the capsid shell. In ΦX174, the N-terminus of the capsid protein resides directly atop the 3-fold axes of symmetry; however, the ΦEC6098 N-terminus stretched across the inner surface of the capsid shell, reaching nearly to the 5-fold axis of the neighboring pentamer. Thus, this extended N-terminus interconnected pentamers on the inside of the capsid shell, presumably promoting capsid assembly, a function performed by the ΦX174 external scaffolding protein. There were also key differences between the ΦX174-like DNA-binding J proteins and its ΦEC6098 homologue VP8. As seen with the J proteins, C-terminal VP8 residues were bound into a pocket within the major capsid protein; however, its N-terminal residues were disordered, likely due to flexibility. We show that the combined location and interaction of VP8’s C-terminus and a portion of VP1’s N-terminus are reminiscent of those seen with the ΦX174 and α3 J proteins. IMPORTANCE There is a dramatic structural and morphogenetic divide within the Microviridae. The well-studied ΦX174-like viruses have prominent spikes at their icosahedral vertices, which are absent in gokushoviruses. Instead, gokushovirus major coat proteins form extensive mushroom-like protrusions at the 3-fold axes of symmetry. In addition, gokushoviruses lack an external scaffolding protein, the more critical of the two ΦX174 assembly proteins, but retain an internal scaffolding protein. The ΦEC6098 virion suggests that key external scaffolding functions are likely performed by coat protein domains unique to gokushoviruses. Thus, within one family, different assembly paths have been taken, demonstrating how a two-scaffolding protein system can evolve into a one-scaffolding protein system, or vice versa.

Parvovirus-derived endogenous viral elements (EVEs) have been found in the genomes of many different animal species, resulting from integration events that may have occurred from more than 50 million years ago to much more recently. Here, we further investigate the properties of autonomous parvovirus EVEs and describe their relationships to contemporary viruses. While we did not find any intact capsid protein open reading frames in the integrated viral sequences, we examined three EVEs that were repaired to form full-length sequences with relatively few changes. These sequences were found in the genomes of Rattus norvegicus (brown rat), Mus spretus (Algerian mouse), and Apodemus sylvaticus (wood mouse). The R. norvegicus sequence was not present in the genomes of the closely related species R. rattus, R. tanezumi, R. exulans, and R. everetti, indicating that it was less than 2 million years old, and the M. spretus and A. sylvaticus sequences were not found in the published genomes of other mouse species, also indicating relatively recent insertions. The M. spretus VP2 sequence assembled into capsids, which had high thermal stability, bound the sialic acid N-acetylneuraminic acid, and entered murine L cells. The 3.89-A structure of the M. spretus virus-like particles (VLPs), determined using cryo-electron microscopy, showed similarities to rodent and porcine parvovirus capsids. The repaired VP2 sequences from R. norvegicus and A. sylvaticus did not assemble as first prepared, but chimeras combining capsid surface loops from R. norvegicus with canine parvovirus assembled, allowing some of that capsid’s structures and functions to be examined. IMPORTANCE Parvovirus endogenous viral elements (EVEs) that have been incorporated into the genomes of different animals represent remnants of the DNA sequences of ancient viruses that infected the ancestors of those animals millions of years ago, but we know little about their properties or how they differ from currently circulating parvoviruses. By expressing the capsid proteins of different parvovirus EVEs that were found integrated into the genomes of three different rodents, we can examine their structures and functions. A VP2 (major capsid protein) EVE sequence from a mouse genome assembled into capsids that had a similar structure and biophysical properties to extant parvoviruses and also bound sialic acids and entered rodent cells. Chimeras formed from combinations of canine parvovirus and portions of the parvovirus sequences from the brown rat genome allowed us to examine the structures and functions of the surface loops of that EVE capsid.

Many entero-, parecho-, and rhinoviruses use immunoglobulin (Ig)-like receptors that bind into the viral canyon and are required to initiate viral uncoating during infection. However, some of these viruses use an alternative or additional receptor that binds outside the canyon. Both the coxsackievirus-adenovirus receptor (CAR), an Ig-like molecule that binds into the viral canyon, and decay-accelerating factor (DAF) have been identified as cellular receptors for coxsackievirus B3 (CVB3). A cryoelectron microscopy reconstruction of a variant of CVB3 complexed with DAF shows full occupancy of the DAF receptor in each of 60 binding sites. The DAF molecule bridges the canyon, blocking the CAR binding site and causing the two receptors to compete with one another. The binding site of DAF on CVB3 differs from the binding site of DAF on the surface of echoviruses, suggesting independent evolutionary processes.

Echovirus 7 (EV7) belongs to the Enterovirus genus within the family Picornaviridae . Many picornaviruses use IgG-like receptors that bind in the viral canyon and are required to initiate viral uncoating during infection. However, in addition, some of the enteroviruses use an alternative or additional receptor that binds outside the canyon. Decay-accelerating factor (DAF) has been identified as a cellular receptor for EV7. The crystal structure of EV7 has been determined to 3.1-Å resolution and used to interpret the 7.2-Å-resolution cryo-electron microscopy reconstruction of EV7 complexed with DAF. Each DAF binding site on EV7 is near a 2-fold icosahedral symmetry axis, which differs from the binding site of DAF on the surface of coxsackievirus B3, indicating that there are independent evolutionary processes by which DAF was selected as a picornavirus accessory receptor. This suggests that there is an advantage for these viruses to recognize DAF during the initial process of infection.

Pleconaril is a broad-spectrum antirhinovirus and antienterovirus compound that binds into a hydrophobic pocket within viral protein 1, stabilizing the capsid and resulting in the inhibition of cell attachment and RNA uncoating. When crystals of human rhinovirus 16 (HRV16) and HRV14 are incubated with pleconaril, drug occupancy in the binding pocket is lower than when pleconaril is introduced during assembly prior to crystallization. This effect is far more marked in HRV16 than in HRV14 and is more marked with pleconaril than with other compounds. These observations are consistent with virus yield inhibition studies and radiolabeled drug binding studies showing that the antiviral effect of pleconaril against HRV16 is greater on the infectivity of progeny virions than the parent input viruses. These data suggest that drug integration into the binding pocket during assembly, or at some other late stage in virus replication, may contribute to the antiviral activity of capsid binding compounds.

Intercellular adhesion molecule 1 (ICAM-1) is the cellular receptor for the major group of human rhinovirus serotypes, including human rhinovirus 14 (HRV14) and HRV16. A naturally occurring variant of ICAM-1, ICAM-1 Kilifi , has altered binding characteristics with respect to different HRV serotypes. HRV14 binds to ICAM-1 only transiently at physiological temperatures but forms a stable complex with ICAM-1 Kilifi . Conversely, HRV16 forms a stable complex with ICAM-1 but does not bind to ICAM-1 Kilifi . The three-dimensional structures of HRV14 and HRV16, complexed with ICAM-1, and the structure of HRV14, complexed with ICAM-1 Kilifi , have been determined by cryoelectron microscopy (cryoEM) image reconstruction to a resolution of approximately 10 Å. Structures determined by X-ray crystallography of both viruses and of ICAM-1 were fitted into the cryoEM density maps. The interfaces between the viruses and receptors contain extensive ionic networks. However, the interactions between the viruses and ICAM-1 Kilifi contain one less salt bridge than between the viruses and ICAM-1. As HRV16 has fewer overall interactions with ICAM-1 than HRV14, the absence of this charge interaction has a greater impact on the binding of ICAM-1 Kilifi to HRV16 than to HRV14.

Structures of all three poliovirus (PV) serotypes (PV1, PV2, and PV3) complexed with their cellular receptor, PV receptor (PVR or CD155), were determined by cryoelectron microscopy. Both glycosylated and fully deglycosylated CD155 exhibited similar binding sites and orientations in the viral canyon for all three PV serotypes, showing that all three serotypes use a common mechanism for cell entry. Difference maps between the glycosylated and deglycosylated CD155 complexes determined the sites of the carbohydrate moieties that, in turn, helped to verify the position of the receptor relative to the viral surface. The proximity of the CD155 carbohydrate site at Asn105 to the viral surface in the receptor-virus complex suggests that it might interfere with receptor docking, an observation consistent with the properties of mutant CD155. The footprints of CD155 on PV surfaces indicate that the south rim of the canyon dominates the virus-receptor interactions and may correspond to the initial CD155 binding state of the receptor-mediated viral uncoating. In contrast, the interaction of CD155 with the north rim of the canyon, especially the region immediately outside the viral hydrophobic pocket that normally binds a cellular “pocket factor,” may be critical for the release of the pocket factor, decreasing the virus stability and hence initiating uncoating. The large area of the CD155 footprint on the PV surface, in comparison with other picornavirus-receptor interactions, could be a potential limitation on the viability of PV escape mutants from antibody neutralization. Many of these are likely to have lost their ability to bind CD155, resulting in there being only three PV serotypes.