ИСТИНА

m7G cap at the 5' end of eukaryotic mRNAs is a key determinant of translational efficiency since it facilitates the first step of translation initiation - loading of ribosomes and initiation factors at the 5'end of mRNA. The ribosome with initiation factors then unwinds the secondary structures and moves along the 5'UTR to locate the initiation codon. It is currently believed that the cap is more essential for the translation of mRNAs with long and structured 5'UTRs. However, it has been shown previously (Andreev et al, NAR 2009) that the 577 nt long 5'UTR of Apaf-1 mRNA can direct translation in a strictly 5' end – dependent mode even in the absence of m7G-cap, and the dependence of its translation on the cap is relatively low. Here, using transfection of cells with capped and uncapped mRNAs we demonstrate that this reduced cap-dependence is determined by at least two 5' proximal structural domains within the Apaf-1 5'UTR. One of these domains alone, domain B, can significantly (6-fold) reduce the cap-dependence when placed within a short 5'UTR of a reporter mRNA. This effect is mostly accounted for by enhanced activity of the hybrid 5' leader in the uncapped form despite the fact that the inserted domain is long and highly structured. Importantly, enzymatic probing of the secondary structure showed that the domain B alone possess the same conformation as in the context of Apaf-1 5'UTR, and the apical Y-shaped stem-loop structure integrity is required for reduced cap-dependece. It has been suggested that the reduced cap-dependence may reflect the property of mRNA to be preferentially translated under stress conditions, so we tested the translation of mRNAs containing the Apaf-1 5'UTR within cells in which apoptosis was induced by treatment with etoposide. As anticipated, the translation of such mRNAs was more resistant to this stress than that of other mRNAs that exhibit a high cap-dependence. Moreover, we show that translation controlled by the Apaf-1 5'UTR is still 5'end dependent even under stress conditions and no internal ribosome entry is involved. These results indicate that the differential contribution of the cap may be a cause of quantitative alterations in the proportion of specific proteins in the cellular proteome under stress conditions. Finally, the implication of specific regulatory structures in 5'UTRs to the cap-independent, but 5'-end dependent translation, so called "5'-CITE" concept (Shatsky et al, Mol Cells 2010) will be discussed.