Abstract
Eukaryotic and archaeal translation initiation complexes have a common structural core comprising e/aIF1, e/aIF1A, the ternary complex (TC, e/aIF2-GTP-Met-tRNAiMet) and mRNA bound to the small ribosomal subunit. e/aIF2 plays a crucial role in this process but how this factor controls start codon selection remains unclear. Here, we present cryo-EM structures of the full archaeal 30S initiation complex showing two conformational states of the TC. In the first state, the TC is bound to the ribosome in a relaxed conformation with the tRNA oriented out of the P site. In the second state, the tRNA is accommodated within the peptidyl (P) site and the TC becomes constrained. This constraint is compensated by codon/anticodon base pairing, whereas in the absence of a start codon, aIF2 contributes to swing out the tRNA. This spring force concept highlights a mechanism of codon/anticodon probing by the initiator tRNA directly assisted by aIF2.
Highlights
Eukaryotic and archaeal translation initiation complexes have a common structural core comprising e/aIF1, e/aIF1A, the ternary complex (TC, e/aIF2-GTP-Met-tRNAiMet) and messenger RNA (mRNA) bound to the small ribosomal subunit. e/aIF2 plays a crucial role in this process but how this factor controls start codon selection remains unclear
We assembled the full initiation complexes (ICs) using archaeal 30S subunits from Pyrococcus abyssi (Pa-30S), initiation factors Pa-aIF1, Pa-aIF1A, the ternary complex (Met-tRNAiMet A1-U72 variant from Escherichia coli (Supplementary Fig. 1a), GDPNP and Pa-aIF2 composed of three subunits a, b and g) and a synthetic 26 nucleotide long mRNA corresponding to the natural start region of the mRNA encoding the elongation factor aEF1A from P. abyssi, which contains a strong Shine-Dalgarno sequence (A( À 17)UUUGGAGGUGAU UUAAA( þ 1)UGCCAAAG(( þ 9))
Comparison of IC0–PREMOTE and IC1–PIN highlights the structural transition that occurs on start codon recognition
Summary
Eukaryotic and archaeal translation initiation complexes have a common structural core comprising e/aIF1, e/aIF1A, the ternary complex (TC, e/aIF2-GTP-Met-tRNAiMet) and mRNA bound to the small ribosomal subunit. e/aIF2 plays a crucial role in this process but how this factor controls start codon selection remains unclear. Recent structural descriptions of eukaryotic translation initiation complexes (ICs) have highlighted some of the structural changes that lead to a closed state with the initiator tRNA base-paired to the start codon[13,14,15,16] These structures have shown how accommodation of the initiator tRNA at the P site, from a POUT to a PIN position[7,17], is stabilized by eIF1A and is accompanied by a closure of the mRNA channel in the 40S subunit as well as by the destabilization of eIF1 binding. Such a parallel relies on the idea that the detection of the start codon in archaea proceeds through a local scanning mechanism within a short mRNA region compatible with maintenance of SD:antiSD pairing This leads to the definition of a structural core, common to eukaryotes and archaea, that controls translation initiation accuracy. According to previous biochemical and genetic studies in eukaryotic systems[7,9,18], the features of the two novel states of the archaeal core IC are likely to be relevant for eukaryotic translation initiation
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
