(A) The EJC-dependent model. In this model, the EJC recruits the NMD factors UPF3X, UPF2, and then UPF1, which is associated with the translation termination complex (eRF1 and eRF3) and with proteins in charge of its phosphorylation (SMG1/8/9). The recruitment of UPF1 is facilitated by the cap bound by the cap-binding proteins (CPB) (CBP80 and CBP20). This interaction is reinforced by the ARS2 protein. (B) The EJC-independent model. In this model, UPF1 concentrates in the 3’UTR of the mRNA, and the translation termination complex is recruited by the UPF3X protein. The cap is bound by the cap binding protein eIF4E.
