Nonsense-mediated decay of an mRNA can be triggered even if the termination codon does not precede an exon junction (Buhler et al. 2006, Eberle et al. 2008, Silva et al. 2008, Singh et al. 2008, Ivanov et al. 2008). UPF1 and PABP seem to modulate the efficiency of translation termination and PABP in the proximity of a termination codon prevents NMD likely by outcompeting UPF1 for interaction with eRF3 (Singh et al. 2008, Ivanov et al. 2008, Silva et al. 2008). Factors in the competition may be the length and secondary structure of the 3' UTR (Buhler et al. 2006, Eberle et al. 2008). UPF1 preferentially binds some but not all longer UTRs (Hogg and Goff 2010).
Interaction of eRF3 with PABP stimulates ribosome dissociation and initiation of a new round of translation on the mRNA. Interaction of eRF3 with UPF1 appears to promote nonsense-mediated decay. It is possible but not yet demonstrated that all components of the SURF complex (SMG1, UPF1, eRF1, eRF3) are assembled on an mRNA without an exon junction complex and that UPF1 is phosphorylated by SMG1.