Smg5 is required for multiple nonsense-mediated mRNA decay pathways in Drosophila
The nonsense-mediated mRNA decay (NMD) pathway is a cellular quality control and post-transcriptional gene regulatory mechanism and is essential for viability in most multicellular organisms. A complex of proteins has been identified to be required for NMD function to occur, however the individual contribution of each of these factors to the NMD process is not well understood. Central to the NMD process are two proteins Upf1 (SMG-2) and Upf2 (SMG-3), which are found in all eukaryotes and are absolutely required for NMD in all organisms in which it has been examined. The other known NMD factors, Smg1, Smg5, Smg6, and Smg7 are more variable in their presence in different orders of organisms, and are thought to have a more regulatory role. Here we present the first genetic analysis of the NMD factor Smg5 in Drosophila. Surprisingly, we find that unlike the other analyzed Smg genes in this organism, Smg5 is essential for NMD activity. We found this is due at least in part to a role for Smg5 in the activity of two separable NMD-target decay mechanisms: endonucleolytic cleavage and 5'-to-3' exonucleolytic decay. Redundancy between these degradation pathways explains why some Drosophila NMD genes are not required for all NMD-pathway activity. We also found that while the NMD component Smg1 has only a minimal role in Drosophila NMD during normal conditions, it becomes essential when NMD activity is compromised by partial loss of Smg5 function. Our findings suggest that not all NMD complex components are required for NMD function at all times, but instead are utilized in a context dependent manner in vivo.