Figure 1.
EJC-dependent nonsense-mediated decay (NMD) [4].
After splicing, exonic-junction complexes (EJCs) remain 20–24 nucleotides upstream of every exon junction. These EJCs are then bound by UPF2, one of the core proteins of NMD. When the first ribosome translates the mRNA, it displaces the EJCs. However, if the ribosome encounters a premature stop codon and stalls, it forms a complex with a downstream EJC, mediated by UPF2 and another complex called SURF. (The SURF complex is named after its constituent proteins [5].) This complex then initiates mRNA decay. Because EJC-dependent NMD requires a downstream EJC, it is only effective in the coding regions upstream of the last EJC (indicated in blue). It cannot detect any premature stop codons downstream of the last EJC (indicated in red). Note that an alternative, less potent mode of NMD takes place in the absence of the EJC [6].