Receptor Priming of Major Group Human Rhinoviruses for Uncoating and Entry at Mild Low-pH Environments

Caliciviridae are small icosahedral positive-sense RNA-containing viruses and include the human noroviruses, a leading cause of infectious acute gastroenteritis and feline calicivirus (FCV), which causes respiratory illness and stomatitis in cats. FCV attachment and entry is mediated by feline junctional adhesion molecule A (fJAM-A), which binds to the outer face of the capsomere, inducing a conformational change in the capsid that may be important for viral uncoating. Here we present the results of our structural investigation of the virus-receptor interaction and ensuing conformational changes. Cryo-electron microscopy and three-dimensional image reconstruction were used to solve the structure of the virus decorated with a soluble fragment of the receptor at subnanometer resolution. In initial reconstructions, the P domains of the capsid protein VP1 and fJAM-A were poorly resolved. Sorting experiments led to improved reconstructions of the FCV-fJAM-A complex both before and after the induced conformational change, as well as in three transition states. These data showed that the P domain becomes flexible following fJAM-A binding, leading to a loss of icosahedral symmetry. Furthermore, two distinct conformational changes were seen; an anticlockwise rotation of up to 15°o f the P domain was observed in the AB dimers, while tilting of the P domain away from the icosahedral 2-fold axis was seen in the CC dimers. A list of putative contact residues was calculated by fitting high-resolution coordinates for fJAM-A and VP1 to the reconstructed density maps, highlighting regions in both virus and receptor important for virus attachment and entry.

Section: Figmentioning

confidence: 69%

The Cryo-Electron Microscopy Structure of Feline Calicivirus Bound to Junctional Adhesion Molecule A at 9-Angstrom Resolution Reveals Receptor-Induced Flexibility and Two Distinct Conformational Changes in the Capsid Protein VP1

Bhella

Goodfellow

2011

“…The uncoating products can be obtained in vitro from virions by receptor binding and/or exposure to low pH (5.5 to 6.0) or elevated temperature (3,(12)(13)(14)(15)(16). Upon receptor binding, the poliovirus converts to the A-particle, which is associated with irreversible conformational changes, including shifts of the capsid protein ␤-barrels and externalization of VP4 and the N terminus of VP1 (8,17,18).…”

mentioning

confidence: 99%

Human Enterovirus 71 Uncoating Captured at Atomic Resolution

Lyu

Ding

Han

et al. 2014

Human enterovirus 71 (EV71) is the major causative agent of severe hand-foot-and-mouth diseases (HFMD) in young children, and structural characterization of EV71 during its life cycle can aid in the development of therapeutics against HFMD. Here, we present the atomic structures of the full virion and an uncoating intermediate of a clinical EV71 C4 strain to illustrate the structural changes in the full virion that lead to the formation of the uncoating intermediate prepared for RNA release. Although the VP1 N-terminal regions observed to penetrate through the junction channel at the quasi-3-fold axis in the uncoating intermediate of coxsackievirus A16 were not observed in the EV71 uncoating intermediate, drastic conformational changes occur in this region, as has been observed in all capsid proteins. Additionally, the RNA genome interacts with the N-terminal extensions of VP1 and residues 32 to 36 of VP3, both of which are situated at the bottom of the junction. These observations highlight the importance of the junction for genome release. Furthermore, EV71 uncoating is associated with apparent rearrangements and expansion around the 2-and 5-fold axes without obvious changes around the 3-fold axes. Therefore, these structures enabled the identification of hot spots for capsid rearrangements, which led to the hypothesis that the protomer interface near the junction and the 2-fold axis permits the opening of large channels for the exit of polypeptides and viral RNA, which is an uncoating mechanism that is likely conserved in enteroviruses. IMPORTANCE Human enterovirus 71 (EV71) is the major causative agent of severe hand-foot-and-mouth diseases (HFMD Human enterovirus 71 (EV71) is the leading causative agent for severe hand-foot-and-mouth diseases (HFMD) in infants and young children (1), and EV71 infection has caused significant morbidity and mortality in the Asia-Pacific regions. During 2008 to 2012, numerous outbreaks of HFMD occurred in mainland China, with over 2,000 fatal cases reported (www.chinacdc.cn). However, vaccines and effective drugs remain unavailable. EV71 belongs to Enterovirus species A within the Enterovirus genus of the Picornaviridae (2). The EV71 virion contains a singlestranded positive-sense RNA genome of 7.5 kb. The viral capsid, which has a diameter of approximately 300 Å, is composed of 60 copies each of VP1 to VP4 (3, 4) that are organized onto a quasi-Tϭ3 icosahedral lattice. The capsid proteins VP1, VP2, and VP3 each possess a ␤-sandwich jelly roll fold and form the outer surface of the capsid shell, whereas VP4 is situated inside the shell. The capsid surface is characterized by a depression encircling each 5-fold symmetry axis, which is referred to as the "canyon" and often contains the receptor-binding site in picornaviruses (5, 6). The hydrophobic pocket of VP1, which is located at the bottom of the canyon, contains a lipid moiety termed the "pocket factor," which likely stabilizes the mature virion. As observed in poliovirus and certain picornaviruses, receptor bindin...

“…Although cell entry of enveloped viruses is well characterized, the mechanisms of viral entry and nuclear targeting of nonenveloped viruses are still poorly understood. For many viruses, cell entry and uncoating are mediated by either receptor (pH independent), low pH, or both (19,30,31,34,39). The endocytic route has two main advantages for karyophilic viruses: one is the efficient and rapid transport toward the perinuclear area, and another is the exposure to a low pH, which can trigger conformational changes required for penetration, uncoating, and nuclear translocation.…”

mentioning

confidence: 99%

Low pH-Dependent Endosomal Processing of the Incoming Parvovirus Minute Virus of Mice Virion Leads to Externalization of the VP1 N-Terminal Sequence (N-VP1), N-VP2 Cleavage, and Uncoating of the Full-Length Genome

Mani

Baltzer

Valle

et al. 2006

100

118

Minute virus of mice (MVM) enters the host cell via receptor-mediated endocytosis. Although endosomal processing is required, its role remains uncertain. In particular, the effect of low endosomal pH on capsid configuration and nuclear delivery of the viral genome is unclear. We have followed the progression and structural transitions of DNA full-virus capsids (FC) and empty capsids (EC) containing the VP1 and VP2 structural proteins and of VP2-only virus-like particles (VLP) during the endosomal trafficking. Three capsid rearrangements were detected in FC: externalization of the VP1 N-terminal sequence (N-VP1), cleavage of the exposed VP2 N-terminal sequence (N-VP2), and uncoating of the full-length genome. All three capsid modifications occurred simultaneously, starting as early as 30 min after internalization, and all of them were blocked by raising the endosomal pH. In particles lacking viral single-stranded DNA (EC and VLP), the N-VP2 was not exposed and thus it was not cleaved. However, the EC did externalize N-VP1 with kinetics similar to those of FC. The bulk of all the incoming particles (FC, EC, and VLP) accumulated in lysosomes without signs of lysosomal membrane destabilization. Inside lysosomes, capsid degradation was not detected, although the uncoated DNA of FC was slowly degraded. Interestingly, at any time postinfection, the amount of structural proteins of the incoming virions accumulating in the nuclear fraction was negligible. These results indicate that during the early endosomal trafficking, the MVM particles are structurally modified by low-pH-dependent mechanisms. Regardless of the structural transitions and protein composition, the majority of the entering viral particles and genomes end in lysosomes, limiting the efficiency of MVM nuclear translocation.