VapC-like PIN domain of yeast exosome subunit Rrp44 endoribonuclease and other eukaryotic homologs.
PIN (PilT N terminus) domain of the Saccharomyces cerevisiae exosome subunit Rrp44 (Ribosomal RNA-processing protein 44 or Protein Dis3 homolog) and other similar eukaryotic homologs are included in this family. The eukaryotic exosome is a conserved macromolecular complex responsible for many RNA-processing and RNA-degradation reactions. It is composed of nine core subunits that directly binds Rrp44. The Rrp44 nuclease is the catalytic subunit of the exosome and has endonuclease activity in the PIN domain and an exoribonuclease activity in its RNase II-like region. Rrp44 binding to the exosome is mediated mainly by the PIN domain and by subunits Rrp41-Rrp45, and binding predictions indicate that the PIN domain active site is positioned on the outer surface of the exosome. This subgroup belongs to the VapC (virulence-associated protein C)-like family of the PIN domain nuclease superfamily. VapC is the PIN-domain ribonuclease toxin from prokaryotic VapBC toxin-antitoxin (TA) systems. VapB is a transcription factor-like protein antitoxin acting as an inhibitor. Other members of the VapC-like nuclease family include FitB toxin of the FitAB TA system, eukaryotic ribonucleases such as Smg6, ribosome assembly factor NOB1, exosome subunit Rrp44 endoribonuclease and rRNA-processing protein Fcf1. The structural properties of the PIN (PilT N terminus) domain indicate its active center, consisting of three highly conserved catalytic residues which coordinate metal ions, in some members, additional metal coordinating residues can be found. Some members of the superfamily lack several of these key catalytic residues. PIN domains within this subgroup contain four of these residues which cluster at the C-terminal end of the beta-sheet and form a negatively charged pocket near the center of the molecule. Recombinant Rrp44 was shown to possess manganese-dependent endonuclease activity in vitro that was abolished by point mutations in these putative metal binding residues of its PIN domain. The PIN active site is geometrically similar in the active center of structure-specific 5' nucleases, PIN-domain ribonucleases of eukaryotic rRNA editing proteins, and bacterial toxins of toxin-antitoxin (TA) operons.
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. In this subgroup 4 conserved catalytic residues are found.
Comment:Recombinant Rrp44 was shown to possess manganese-dependent endonuclease activity in vitro that was abolished by point mutations in the putative metal binding residues of its PIN domain.
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