RAN translation and frameshifting as translational challenges at simple repeats of human neurodegenerative disorders

Wojciechowska, Marzena; Olejniczak, Marta; Galka-Marciniak, Paulina; Jazurek, Magdalena; Krzyzosiak, Wlodzimierz J.

Canonical translation initiation is a complex process requiring more than ten initiation factors ( Figure 2) (23,24) . However, under certain cellular conditions such as viral infection, protein synthesis might be switched to another mode and proceed with minimal involvement of the translation machinery (62) . Depending on the mode of RAN translation initiation, different sets of protein factors may be involved (Figure 2 ). In a cap-dependent scanning mechanism, the utilization of a full set of initiation machinery might be perturbed when the scanning ribosome stalls at a highly structured obstacle of expanded repeats with bound proteins (Figure 2a) . Because this stalling could force the scanning ribosome to pause, various proteins attached to structured RNA and proteins of the scanning complex might be rearranged, triggering noncanonical translation initiation at non-AUG codons. It is plausible that such perturbation could involve the eIF1 initiation factors, including eIF1A, which plays a critical role in the fidelity of start codon selection (23, 24) . Another candidate to consider is eIF2, whose function is to deliver Met-tRNA i Met to the ribosome. There is evidence of eIF2-independent translation under stress conditions (63) (64) (65) (66) (67) . As reported, RAN translation does not require an AUG codon, which implies that tRNAs charged with amino acids other than methionine are involved in translation initiation (14) . Moreover, one of the translation initiation factors, eIF2D, can direct tRNA Phe to the P-site of the ribosome to initiate translation (65) . A distinct set of proteins may be considered in the context of direct recruitment of the ribosome to structures formed by the expanded repeats (Figure 2b ). One possible scenario is that the structured expanded repeats bind the small ribosomal subunit in the absence of initiation factors, but RAN translation still requires eIF3 and eIF2-GTP for initiation, as shown for viral IRES type III (49, 50) . Another scenario involves the steps and factors described for viral IRES type IV (49, 50) , in which RAN protein synthesis is initiated with minimal involvement of the translation machinery, assuming that the structures formed by expanded repeats are tightly folded and structurally mimic the missing initiation factors. In contrast to viral IRES, their cellular counterparts are less understood, more diverse in their structure and less stable (67) . Taken together, the available data remain insufficient to answer the question of whether IRES mechanisms are indeed involved in RAN translation.