DEDDh 3'-5' exonuclease domain of RNA exonuclease 4, XPMC2, Interferon Stimulated Gene product of 20 kDa, and similar proteins
This subfamily is composed of RNA exonuclease 4 (REX4 or Rex4p), XPMC2, Interferon (IFN) Stimulated Gene product of 20 kDa (ISG20), and similar proteins. REX4 is involved in pre-rRNA processing. It controls the ratio between the two forms of 5.8S rRNA in yeast. XPMC2 is a Xenopus gene which was identified through its ability to correct a mitotic defect in fission yeast. The human homolog of XPMC2 (hPMC2) may be involved in angiotensin II-induced adrenal cell cycle progression and cell proliferation. ISG20 is an IFN-induced antiviral exonuclease with a strong preference for single-stranded RNA and minor activity towards single-stranded DNA. These proteins are DEDDh-type DnaQ-like 3'-5' exonucleases containing 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. REX proteins function in the processing and maturation of many RNA species, similar to the function of Escherchia coli RNase T.