DEDDh 3'-5' exonuclease domain of the eukaryotic exoribonuclease PAN2
PAN2 is the catalytic subunit of poly(A) nuclease (PAN), a Pab1p-dependent 3'-5' exoribonuclease which plays an important role in the posttranscriptional maturation of pre-mRNAs. PAN catalyzes the deadenylation of poly(A) tails, which are initially synthesized to default lengths of 70 to 90, to mRNA-specific lengths of 55 to 71. Pab1p and PAN also play a role in the export and decay of mRNA. PAN2 contains a DEDDh-type DnaQ-like 3'-5' exonuclease domain with three conserved sequence motifs termed ExoI, ExoII and ExoIII, with a specific Hx(4)D conserved pattern at ExoIII. These motifs are clustered around the active site and contain four conserved acidic residues that serve as ligands for the two metal ions required for catalysis.