Ensemble cryo EM uncovers inchworm like translocation of a viral IRES through the ribosome
Internal ribosome entry sites (IRESs) mediate cap independent translation of viral mRNAs. Using electron cryo microscopy of a single specimen, we present five ribosome structures formed with the Taura syndrome virus IRES and translocase eEF2*GTP bound with sordarin. The structures suggest a trajectory of IRES translocation, required for translation initiation, and provide an unprecedented view of eEF2 dynamics. The IRES rearranges from extended to bent to extended conformations. This inchworm like movement is coupled with ribosomal inter subunit rotation and 40S head swivel. eEF2, attached to the 60S subunit, slides along the rotating 40S subunit to enter the A site. Its diphthamide bearing tip at domain IV separates the tRNA mRNA like pseudoknot I (PKI) of the IRES from the decoding center. This unlocks 40S domains, facilitating head swivel and biasing IRES translocation via hitherto elusive intermediates with PKI captured between the A and P sites. The structures suggest missing links in our understanding of tRNA translocation. DOI: http:/dx.doi.org/10.7554/eLife.14874.001
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