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

Ping Zhang, Purdue University

Abstract

Poliovirus (PV), the causative agent of paralytic poliomyelitis, is a small non-enveloped virus, with a single strand RNA genome encapsidated in an icosahedral protein capsid. Polioviruses are members of the Enterovirus genus within the Picornaviridae family. Before the development of an effective vaccine, poliovirus was a fearsome pathogen and the focus of intense scientific study. Today, polio is no longer a major threat to health in developed countries, but global efforts to eradicate the disease still face serious obstacles. Despite landmark studies of poliovirus replication, native virion and entry intermediates’ structures, details of poliovirus-receptor interactions remain poorly understood. When poliovirus recognizes its receptor, CD155, the virus changes from a 160S to a 135S particle before releasing its genome into the cytoplasm. A fatty acid-like molecule known as the “pocket factor” was observed to bind into a hydrophobic pocket beneath the canyon in PVs. Binding of CD155 into the canyon probably competes with the binding of the “pocket factor” into the hydrophobic pocket. Release of the pocket factor destabilizes the virus and thereby initiates uncoating. CD155 is a transmembrane protein with three immunoglobulin-like extracellular domains, D1 to D3, where D1 is recognized by the virus. In this thesis I describe the crystal structure determination of D1D2 to 3.5 Å resolution. I also describe the use of cryoEM image reconstruction to analyze the virus-receptor complexes of all three PV serotypes to ∼8.5 Å resolution. Fitting of the atomic structure of CD155 into the EM densities allows the determination of the binding interface between the receptor and the virus. These structures show that, in comparison to human rhinoviruses, the virus-receptor interactions for polioviruses have a greater dependence on hydrophobic interactions, as might be required for a virus that can inhabit environments of different pH. The pocket factor was shown to remain in the virus during the first recognition stage. The present structures, when combined with earlier mutational investigations, show that in the subsequent entry stage the receptor moves further into the canyon when at a physiological temperature, thereby expelling the pocket factor and separating the viral subunits to form 135S particles. These results provide a detailed analysis of how a non-enveloped virus can enter its host cell.

Degree

Ph.D.

Advisors

Rossmann, Purdue University.

Subject Area

Virology

COinS