Fig. 7

Model comparing potential G4C2 RAN-translation mechanisms and canonical translation. a. Ternary complex formation requires eIF5-mediated exchange of GDP to GTP on eIF2 complex (includes eIFs 2α, 2β, 2γ). eIF2α is highly regulated during stress and is reported to mediate G4C2 translation [12, 27]. eIF2β and eIF5 were identified as modifiers in this study. b. In normal translation, the formation of the 43S pre-initiation complex (PIC) involves the joining of a number of factors, including Ternary complex (described in a) and eIFs 1, 1A, 3, and 5. c. A minimal PIC complex may potentially mediate RAN-translation [1, 42, 76, 86]. d. mRNA transcripts are recognized by the eIF4F complex, includes eIFs 4E, 4G, 4A. All of these have been defined as G4C2 translation factors arguing that G4C2 RAN-translation is cap-dependent [12, 84]. eIF4E recognizes the 5-prime m⁷G cap on mRNAs [10, 77, 78]; notably, 4 of 6 eIF4E components were identified in our screen. eIF4A is recruited by eIF4E to mRNA transcripts (via the scaffold protein eIF4G). mRNA is then unwound by eIF4A, an activity that is significantly promoted by eIF4B or eIF4H, identified herein [24, 68, 70, 82, 91]. This action allows for the formation of the 48S scanning complex. e. In canonical translation, the 48S scanning complex moves down a transcript until identifying an AUG start codon. A CUG codon in the LDS sequence upstream of G4C2 may function as a start codon in the GA-reading frame [27, 84]. Frame-shifting could allow for translation of the GR and GP from this codon. Candidate RAN translation factors eIF5B, and potentially eIF5, mediate ribosome scanning, start codon recognition, and translation activation [10, 45, 61]. We note that mechanisms underlying RAN-translation are still relatively unknown. This model is based on current literature and canonical functions of translation factors