ИСТИНА

During eukaryotic translation initiation, 43S ribosomal complex bearing Met-tRNAi scans an mRNA’s leader unless an AUG codon in an appropriate context is found. Establishment of the stable codon-anticodon base pairing traps the ribosome on the initiator codon and triggers rearrangements of the complex which lead to Pi release from eIF2-bound GTP. According to the generally accepted view, AUG recognition by the scanning 43S complex sets the final point in a process of start codon selection, while latter stages do not contribute to this process. Using cell-free translation system and translation reconstitution approach combined with kinetic toe-printing assay we show here that after the 48S complex is formed on a AUG codon under conditions when GTP hydrolysis is impaired, the ribosomal subunit is able to resume scanning and slides downstream to the next initiation codons. In contrast to conventional leaky scanning, such sliding only occurs after a relatively long pause at the previously recognized AUG. Thus, recognition of an AUG per se does not directly lead to selection of this codon for initiation of protein synthesis. Instead, it is eIF5-induced GTP hydrolysis leading to eIF2-GDP dissociation that irreversibly traps the 48S complex, and such a complex is further stabilized by eIF5B and 60S joining.