VapC-like PIN domain of human nonsense-mediated decay factor Smg5, and other similar eukaryotic homologs
Nonsense-mediated decay (NMD) factors, Smg5 and Smg6 are essential to the post-transcriptional regulatory pathway, NMD, which recognizes and rapidly degrades mRNAs containing premature translation termination codons. The PIN (PilT N terminus) domain belongs to a large nuclease superfamily. The structural properties of the PIN domain indicate its putative active center, consisting of invariant acidic amino acid residues (putative metal-binding residues), is geometrically similar in the active center of structure-specific 5' nucleases (also known as Flap endonuclease-1-like), PIN-domain ribonucleases of eukaryotic rRNA editing proteins, and bacterial toxins of toxin-antitoxin (TA) operons. Point mutation studies of the conserved aspartate residues in the catalytic center of the Smg6 PIN domain revealed that Smg6 is the endonuclease involved in human NMD. However, Smg5 lacks several of these key catalytic residues and does not degrade single-stranded RNA, in vivo.
Comment:based on structure and experimental evidence from PIN-superfamily members belonging to the FEN-like, VapC-like, PRORP-like, and LabA-like families
Comment:The PIN domain superfamily contains three highly conserved catalytic residues which coordinate metal ions; in some members, additional metal coordinating residues can be found while some others lack several of these key catalytic residues. For a few members of this subgroup 5 conserved catalytic residues are found.
Comment:Note that SMG5, a member of this subgroup, lacks all but a single aspartic acid residue in the active site and consequently is inactive as a nuclease
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