Structures of the PIN domains of SMG6 and SMG5 reveal a nuclease within the mRNA surveillance complex

EMBO J. 2006 Nov 1;25(21):5117-25. doi: 10.1038/sj.emboj.7601377. Epub 2006 Oct 19.

Abstract

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.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Carrier Proteins / genetics
  • Carrier Proteins / metabolism*
  • Codon, Nonsense / metabolism*
  • Drosophila
  • Drosophila Proteins / genetics
  • Drosophila Proteins / metabolism
  • Humans
  • Multiprotein Complexes / genetics
  • Multiprotein Complexes / metabolism
  • Protein Biosynthesis / genetics
  • Protein Structure, Tertiary / genetics
  • RNA Stability / physiology*
  • Ribonuclease H / genetics
  • Ribonuclease H / metabolism*
  • Telomerase / genetics
  • Telomerase / metabolism*

Substances

  • Carrier Proteins
  • Codon, Nonsense
  • Drosophila Proteins
  • Multiprotein Complexes
  • SMG5 protein, human
  • SMG6 protein, human
  • Telomerase
  • Ribonuclease H