DEDDh 3'-5' exonuclease domain of three prime repair exonuclease (TREX)1, TREX2, and similar proteins
Three prime repair exonuclease (TREX)1 and TREX2 are closely related DEDDh-type DnaQ-like 3'-5' exonucleases. They contain three conserved sequence motifs known as ExoI, II, and III, with a specific Hx(4)D conserved pattern at ExoIII. These motifs contain four conserved acidic residues that participate in coordination of divalent metal ions required for catalysis. Both proteins play a role in the metabolism and clearance of DNA. TREX1 is the major 3'-5' exonuclease activity detected in mammalian cells. Mutations in the human TREX1 gene can cause Aicardi-Goutieres syndrome (AGS), which is characterized by perturbed innate immunity and presents itself as a severe neurological disease. TREX1 degrades ssDNA generated by aberrant replication intermediates to prevent checkpoint activation and autoimmune disease. There are distinct structural differences between TREX1 and TREX2 that point to different biological roles for these proteins. The main difference is the presence of about 70 amino acids at the C-terminus of TREX1. In addition, TREX1 has a nonrepetitive proline-rich region that is not present in the TREX2 protein. Furthermore, TREX2 contains a conserved DNA binding loop positioned adjacent to the active site that has a sequence distinct from the corresponding loop in TREX1. Truncations in the C-terminus of human TREX1 cause autosomal dominant retinal vasculopathy with cerebral leukodystrophy (RVCL), a neurovascular syndrome featuring a progressive loss of visual acuity combined with a variable neurological picture.