FIGURE 1.

Nonsense mediated decay of mRNAs. The monitoring of mRNAs for premature termination codons (PTCs) is mediated by ribosomes during the “pioneering round of translation.” Several proteins associate with sequences near exon/intron borders of the spliced mRNA, forming the exon junction complex (EJC). Some EJC proteins are removed during export whereas others are removed during translation and thus serve as indicators of whether a particular region of the mRNA has been translated. (A) If no EJC proteins are on the mRNA when the pioneering ribosome encounters a chain termination codon, this codon is read as a normal termination signal, at the 3′ end of the coding region. (B) If the ribosome has not traversed all exon/intron borders, EJC protein(s) remain on the mRNA and serve as an indication that any termination codon encountered is a PTC. As a consequence, nonsense-mediated decay (NMD) is initiated, resulting in destruction of the mRNA. The pioneering round of translation is shown here as happening in the cytoplasm (or the perinuclear cytoplasm), and with the released proteins being imported into the nucleus.