Involved in nonsense-mediated decay (NMD) of mRNAs containing premature stop codons. Is recruited by release factors to stalled ribosomes together with smg1 and smg8 (forming the SMG1C protein kinase complex) and, in the SMG1C complex, is required for the efficient association between smg1 and smg8 (By similarity). Component of the SMG1C complex composed of smg1, smg8 and smg9. Belongs to the SMG9 family.