Figure 7

Effects of small, ‘local’, GC-rich mRNA stem-loops on the yeast 43S PIC. Protein translation in eukaryotes starts (1) with the binding of the eukaryotic initiation factor 4F (eIF4F = eIF4E and eIF4G) to the 5′ cap (5′ m⁷G; black dot) of the mature mRNA. eIF4A together with eIF4B are thought to unwind secondary structure in an ATP-dependent fashion close to the 5′ cap to allow access for the 43S PIC (small grey ovals). The 43S PIC consists of the 40S ribosomal subunit to which eIF1, 1A, 2 (bound to GTP), 3 and 5 and tRNA^Met are bound. After attachment of the 43S PIC next to the 5′ cap region AUG start-codon scanning proceeds. Recognition of the AUG start-codon (2) induces GTP hydrolysis and the release of eIF2, GDP, and Pi, which is followed by the eIF5B-GTP catalyzed joining of the 60S subunit (large grey oval) and the displacement of eIF5, eIF1 and eIF3 followed by hydrolysis of eIF5B-GTP and the release of eIF5B-GDP and eIF1A leading to the production of an 80S initiation complex competent for elongation (3) [36, 37]. Small, ‘local’, GC-rich mRNA stem-loops 5′ proximal to the AUG start codon inhibit AUG start-codon scanning of the yeast 43S PIC by providing a physical barrier to the advancing complex and possibly get trapped inside the complex (center left; [36, 40]). The ratio between mRNA molecules that contain a GC-rich mRNA stem-loop (left side) in front of the advancing 43S PIC and those that don’t (right side) may be a function of the thermodynamic stability of stem-loops and determine the amount of protein that can be translated leading to a biphasic polysome distribution (the majority of mRNA molecules bound to 43S PIC (center left) and a small pool of mRNA that is inversely proportional to the stability of the stem-loop and heavily loaded with actively translating 80S ribosomes (bottom)).