Kinetic Analysis of the Effect of Poliovirus Receptor on Viral Uncoating: the Receptor as a Catalyst

Abstract

We examined the role of soluble poliovirus receptor on the transition of native poliovirus (160S or N particle) to an infectious intermediate (135S or A particle). The viral receptor behaves as a classic transition state theory catalyst, facilitating the N-to-A conversion by lowering the activation energy for the process by 50 kcal/mol. In contrast to earlier studies which demonstrated that capsid-binding drugs inhibit thermally mediated N-to-A conversion through entropic stabilization alone, capsid-binding drugs are shown to inhibit receptor-mediated N-to-A conversion through a combination of enthalpic and entropic effects.Poliovirus is a nonenveloped virus of the family Picornaviridae. Picornaviruses share an icosahedral capsid architecture consisting of 60 copies of four proteins, VP1, VP2, VP3, and VP4. The surface of the virion is dominated by prominent star-shaped mesas at the fivefold axes and three-bladed propeller-like features at the threefold axes. These surface features are separated by deep canyons encircling the fivefold axes. These canyons are involved in many essential aspects of capsid function. Structural studies have shown that the receptor footprints for major group rhinoviruses (19) and poliovirus (2, 10, 27) map to the canyon. At the base of the canyon underneath the receptor footprint, there is an entry to a long, narrow hydrophobic pocket within the ␤-barrel core of VP1. For most entero-and rhinoviruses, crystallographic studies have revealed that this pocket is occupied by an unidentified fatty acid-like moiety, or pocket factor (6,12,17,18,23), which can be displaced by a family of capsid-binding antiviral drugs (9,22). Interestingly, the pocket factor and the antiviral drugs can exert large-scale, global effects on the capsid's conformational dynamics, which play a critical role in the viral life cycle.When poliovirus attaches to its receptor, the particle converts irreversibly from the N (native or 160S) to the A (infectious [4] intermediate, or 135S) conformation. In the course of this uncoating transition, normally internal components, including VP4 and the N-terminal extension of VP1, are externalized. Externalization of these components has been shown to facilitate the attachment of the A particle to liposomes in vitro (7), suggesting a mechanism for the entry of virus or viral RNA to the cell (1, 2). Transient and reversible exposure of portions of VP4 and the N terminus of VP1 also occurs naturally at physiological temperatures (14). This "breathing" process suggests that the particle is primed to undergo the N-to-A transition but cannot complete the transition in the absence of a trigger, i.e., the receptor. We have previously proposed that the receptor acts like an enzyme, accelerating the rate of the N-to-A transition at physiological temperature by lowering the activation energy (E a ) for the transition. Later in the cell entry process, the A particle undergoes further changes, which result in the release of the viral RNA and formation of an empty particle that sediments at...

Observed Hysteresis of Virus Capsid Disassembly Is Implicit in Kinetic Models of Assembly

“…Hysteresis can mask the underlying fragility of a virus. A particularly revealing observation that supports this argument is that poliovirus receptor acts as transition state catalyst to lower the activation energy for conversion of 160 S poliovirus to an infectious intermediate (35). This final point also suggests that where there is hysteresis, a trigger may be required for uncoating, giving the virus an important regulatory control.…”

Crystal structure of CD155 and electron microscopic studies of its complexes with polioviruses

“…Thus, the results presented here probably relate to early events of CD155 binding before the expulsion of the pocket factor. Furthermore, it has been shown that binding of CD155 to PVs is a 2-step process (44,45). Thus, presumably, the present EM studies represent the first of these steps where the release of free energy when CD155 binds to the virus is insufficient to overcome the energy barrier between the initial recognition event and the formation of the subsequent intermediates in the infection process.…”

Poliovirus Cell Entry: Common Structural Themes in Viral Cell Entry Pathways

“…The native-to-A particle conversion can also be induced by solubilized forms of the receptor (40,55) and by the soluble ectodomain of the receptor in the absence of cells (3,111). The A particle has altered sedimentation behavior (sedimenting at 135S versus 160S for the native virion) and altered antigenicity.…”

“…The data also demonstrate that the receptor lowers the activation barrier for the reaction by nearly 50 kcal/mole. Thus, the receptor behaves like a classical transition-state catalyst (111).…”