Translational controls impinging on the 5'-untranslated region and initiation factor proteins.

Translation of eukaryotic mRNAs is generally initiated by the scanning ribosome mechanism. This can be downregulated by high affinity protein binding to cap-proximal RNA motifs. Translation can also be regulated by short open reading frames within the 5' -untranslated region. A key factor for initiation is elF4F, in which one of the polypeptide chains, elF4G, seems to have a bridging function and binds three other factors at separate sites: elF4E (the cap-binding factor), the helicase elF4A, and elF3, which also interacts with 40S ribosomal subunits. Initiation is regulated by the MAP kinase and rapamycin-sensitive signalling pathways, which control phosphorylation of elF4E and 4E-BP1, a protein which in the dephosphorylated form binds and sequesters elF4E.