Inhibition of ricin A‐chain (RTA) catalytic activity by a viral genome‐linked protein (VPg).

Abstract

Ricin is a plant derived protein toxin produced by the castor bean plant (Ricinus communis). The Centers for Disease Control (CDC) classifies ricin as a Category B biological agent. Currently, there is neither an effective vaccine that can be used to protect against ricin exposure nor a therapeutic to reverse the effects once exposed. Here we quantitatively characterize interactions between catalytic ricin A‐chain (RTA) and a viral genome‐linked protein (VPg) from turnip mosaic virus (TuMV). VPg and its N‐terminal truncated variant, VPg1‐110, bind to RTA and abolish ricin’s catalytic depurination of 28S rRNA in vitro and in a cell‐free rabbit reticulocyte translational system. RTA and VPg bind in a 1 to 1 stoichiometric ratio, and their binding affinity increases ten‐fold as temperature elevates (5 °C to 37 °C). RTA‐VPg binary complex formation is enthalpically driven and favored by entropy, resulting in an overall favorable energy, ΔG = −136.8 kJ/mol. Molecular modeling supports our experimental observations and predicts a major contribution of electrostatic interactions, suggesting an allosteric mechanism of downregulation of RTA activity through conformational changes in RTA structure, and/or disruption of binding with the ribosomal stalk. Fluorescence anisotropy studies show that heat affects the rate constant and the activation energy for the RTA‐VPg complex, Ea = −62.1 kJ/mol. The thermodynamic and kinetic findings presented here are an initial lead study with promising results and provides a rational approach for synthesis of therapeutic peptides that successfully eliminate toxicity of ricin, and other cytotoxic RIPs.