Eukaryotic mRNAs containing premature translation termination codons (PTCs) are rapidly degraded by a process termed "nonsense-mediated mRNA decay" (NMD). We examined protein:protein and protein:RNA interactions among C. elegans proteins required for NMD. SMG-2, SMG-3, and SMG-4 are orthologs of yeast and mammalian Upf1, Upf2, and Upf3, respectively. A combination of immunoprecipitation and yeast two-hybrid experiments indicates that SMG-2 interacts with SMG-3, SMG-3 interacts with SMG-4, and SMG-2 interacts indirectly with SMG-4 via shared interactions with SMG-3. Such interactions are similar to those observed in yeast and mammalian cells. SMG-2, -3, and -4 interactions require neither SMG-2 phosphorylation, which is abolished in
smg-1 mutants, nor SMG-2 dephosphorylation, which is reduced or eliminated in
smg-5 mutants. SMG-2 preferentially associates with PTC-containing mRNAs. We monitored association of SMG-2, SMG-3 and SMG-4 with mRNAs of five endogenous genes whose mRNAs are alternatively spliced to either contain or not contain PTCs. SMG-2 associates with both PTC-free and PTC-containing mRNPs, but it strongly and preferentially associates with ("marks") those containing PTCs. SMG-2 marking of PTC-mRNPs is enhanced by SMG-3 and SMG-4, but SMG-3 and SMG-4 are not detectably associated with those same mRNPs. Neither SMG-2 phosphorylation nor dephosphorylation is required for selective association of SMG-2 with PTC-containing mRNPs, indicating that SMG-2 is phosphorylated only after premature terminations have been discriminated from normal terminations. We discuss these observations with regard to the functions of SMG-2 and its phosphorylation during NMD.