ИСТИНА

During translation, aminoacyl-tRNAs are delivered to the ribosome by specialized GTPases called translation factors. Here we report the tRNA binding to the P-site of the 40S ribosome by a novel GTP-independent factor eIF2D. The binding of initiator tRNA only occurred when the AUG codon is placed precisely into the P-site of the 40S ribosomal subunit, the situation that takes place during initiation complex formation on the HCV IRES or on some other specific RNAs (leaderless mRNA and A-rich mRNAs with relaxed scanning dependence). The authenticity of resulting 48S initiation complex was confirmed by both toe-printing and sensitivity of the complex to bacterial toxin RelE (1). However, the factor did not support scanning and significant Met-tRNA binding with the beta-globin mRNA, a conventional cap-dependent mRNA. Interestingly, its activity in tRNA binding with 40S subunits did not require the presence of the aminoacyl moiety. Moreover, the factor possessed an unique ability to deliver non-Met (elongator) tRNAs into the P-site of the 40S subunit. The properties and activity of this factor resembled that of factor IF-M1 studied in early 1970s (2). We were able to purify this protein from both RRL and HeLa cells extract and identify it by mass spectrometry fingerprinting. The identity of the human factor was confirmed by similar functional properties of the corresponding recombinant protein. The protein is encoded by a single gene present in the genomes of all eukaryotes and consists of two functional domains. One of them is a well-known SUI1 domain present also in translation initiation factor eIF1. In yeast, the protein has been shown to be associated with the translation machinery and necessary for the proper sporulation process. In summary, we present a novel translation factor which is able to deliver tRNA into the P-site of a pre-formed 40S-mRNA complex in a GTP-independent manner. Possible functions of the factor in eukaryotic translation initiation and/or elongation will be discussed.