The NMD gene Smg5 is required for viability independent of NMD function. Jonathan Nelson¹, Dominique Foerster², Stefan Luschnig², Mark Metzstein¹. 1) Human Genetics, University of Utah, Salt Lake City, UT; 2) University of Zurich IMLS, Zurich, Switzerland.

   Nonsense mediated mRNA decay (NMD) is a cellular quality control mechanism that targets mRNAs containing premature termination codons for degradation. The NMD pathway is tightly regulated, particularly at the transition from target recognition to decay initiation. This transition is in part regulated by phosphorylation of the key NMD factor Upf1 by the kinase Smg1. Phosphorylation of Upf1 provides platforms for the endonuclease Smg6, which initiates degradation of the target mRNA, and Smg5, which recruits a phosphatase that initiates dephosphorylation of Upf1 and release of the NMD complex from the mRNP.
   Our current understanding of the role of Upf1 phosphorylation in NMD function comes from cell culture experiments. However, less is known about the NMD requirements of Upf1 modification in vivo. Previous work in Drosophila has shown that Upf1 mutants are lethal, probably due to severe defects in NMD function, while Smg1 mutants have only minor NMD defects and are viable. We have now found that loss-of-function alleles of Smg5 are lethal. One possible explanation is that Smg5 lethality is due to hyperphosphorylation of Upf1 by Smg1, caused by loss of the Smg5-recruited phosphatase, and the subsequent failure of the NMD complex to disassemble and recycle. However, we have found that Smg1-Smg5 double mutants are still lethal. These data suggest two alternative hypotheses: 1) Smg5-mutant hyperphosphorylation of Upf1 can occur through another kinase, or 2) an NMD-independent function of Smg5 is required for viability. We found that Smg5 mutants have only weak defects in NMD function, suggesting that lethality of Smg5 single and Smg1-Smg5 double mutants is not due to defects in NMD function, but instead due to an unknown NMD-independent function of Smg5. We are currently working to elucidate the contribution of Smg5 to Upf1 phosphorylation status, NMD activity, as well as other NMD-independent functions of Smg5 that may be required for viability.