Structures of the PIN domains of SMG6 and SMG5 reveal a nuclease within the mRNA surveillance complex.
SMG6 and SMG5 are essential factors in nonsense-mediated mRNA decay, a conserved pathway that degrades mRNAs with premature translation termination codons. Both SMG5 and SMG6 have been predicted to contain a C-terminal PIN (PilT N-terminus) domain, present in proteins with ribonuclease activity. We have determined the structures of human SMG5 and SMG6 PIN domains. Although they share a similar overall fold related to ribonucleases of the RNase H family, they have local differences at the putative active site. SMG6 has the canonical triad of acidic residues that are crucial in RNase H for nuclease activity, while SMG5 lacks key catalytic residues. The structural differences are reflected at the functional level. Only the PIN domain of SMG6 has degradation activity on single-stranded RNA in vitro. This difference in catalytic activity is conserved in Drosophila, where an SMG6 with an inactive PIN domain inhibits NMD in a dominant-negative manner. Our findings suggest that the NMD machinery has intrinsic nuclease activity that is likely to contribute to the rapid decay of mRNAs that terminate translation prematurely.
Glavan
Filip
F
European Molecular Biology Laboratory, Heidelberg, Germany.
Behm-Ansmant
Isabelle
I
Izaurralde
Elisa
E
Conti
Elena
E
D000818:Animals; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D018389:Codon, Nonsense / Q000378:metabolism*; D004330:Drosophila; D029721:Drosophila Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D046912:Multiprotein Complexes / Q000235:genetics / Q000378:metabolism; D014176:Protein Biosynthesis / Q000235:genetics; D017434:Protein Structure, Tertiary / Q000235:genetics; D020871:RNA Stability / Q000502:physiology*; D016914:Ribonuclease H / Q000235:genetics / Q000378:metabolism*; D019098:Telomerase / Q000235:genetics / Q000378:metabolism*; D004722:Endoribonucleases
D002352:Carrier Proteins; D018389:Codon, Nonsense; D029721:Drosophila Proteins; D046912:Multiprotein Complexes; D016914:Ribonuclease H; D019098:Telomerase; C515801:SMG5 protein, human; C512261:SMG6 protein, human; D004722:Endoribonucleases