Keywords:

Abstract:

<p>The genomes of several plant viruses contain RNA structures at their 3&#39; ends called cap-independent translation enhancers (CITEs) that bind the host protein factors such as mRNA 5&#39; cap-binding protein eIF4E for promoting cap-independent genome translation. However, the structural basis of such 5&#39; cap-binding protein recognition by the uncapped RNA remains largely unknown. Here, we have determined the crystal structure of a 3&#39; CITE, panicum mosaic virus-like translation enhancer (PTE) from the saguaro cactus virus (SCV), using a Fab crystallization chaperone. The PTE RNA folds into a three-way junction architecture with a pseudoknot between the purine-rich R domain and pyrimidine-rich Y domain, which organizes the overall structure to protrude out a specific guanine nucleotide, G18, from the R domain that comprises a major interaction site for the eIF4E binding. The superimposable crystal structures of the wild-type, G18A, G18C, and G18U mutants suggest that the PTE scaffold is preorganized with the flipped-out G18 ready to dock into the eIF4E 5&#39; cap-binding pocket. The binding studies with wheat and human eIF4Es using gel electrophoresis and isothermal titration calorimetry, and molecular docking computation for the PTE-eIF4E complex demonstrated that the PTE structure essentially mimics the mRNA 5&#39; cap for eIF4E binding. Such 5&#39; cap mimicry by the uncapped and structured viral RNA highlights how viruses can exploit RNA structures to mimic the host protein-binding partners and bypass the canonical mechanisms for their genome translation, providing opportunities for a better understanding of virus-host interactions and non-canonical translation mechanisms found in many pathogenic RNA viruses.</p>