Serine/threonine protein kinase involved in mRNA surveillance. Recognizes the substrate consensus sequence [ST]-Q. Involved in nonsense-mediated decay (NMD) of mRNAs containing premature stop codons, probably by phosphorylating Upf1. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Component of a post-splicing multiprotein NMD complex. Belongs to the PI3/PI4-kinase family. According to some authors (PubMed:15965240) it may not be directly involved in NMD, as mutants do not abolish NMD. However, other data clearly show its involvement in NMD (PubMed:12881430 and PubMed:16199763).