TREX1 was shown to bind and degrade the HIV DNA fragments, which were generated during reverse transcription in HIV-infected human cells (Yan N et al. 2010). Other studies showed that TREX1 may regulate host responses to infection with several different types of RNA viruses (Hasan M et al. 2012). TREX1 is thought to clear viral derived DNA from the cytoplasm and thereby inhibit the activation of cytosolic DNA sensors (Yan N et al. 2010; Hasan M et al. 2012).
Structural studies of the human and mouse TREX proteins revealed the dimeric nature of the TREX family exonucleases (Brucet M et al. 2007; de Silva U et al. 2007, 2009; Perrino FW et al. 2005; Bailey SL et al 2012). Besides, the stable TREX1 dimer was purified from bacterial cells expressing affinity-tagged human TREX1 proteins (Orebaugh CD et al. 2011).Comparative structural analysis of wild type (wt) and natural mutant variants of TREX1 in complex with ssDNA provided some insights into mechanism of the TREX1 exonuclease activity (Bailey SL et al 2012). The reaction begins with the binding of metal ions and DNA substrate in the enzyme active site, which results in the transition of catalytic histidine residue H195 from a disordered to an ordered state. The distance between two divalent metal ions is also essential for catalytic activity. The authors proposed a mechanism where the two protomers in TREX1 dimer alternate back and forth between active and resting states as they degrade substrate. The activity status is mediated by the dual conformation of H195, which is coordinated with the shift of the metal ion from 3.1 A when H195 is out of the active site (resting) to 3.6 A when H195 moves into the active site (active) (Bailey SL et al 2012). In addition, the structures of the TREX1 mutant proteins (dominant D200H, D200N and D18N homodimer mutants derived from AGS and FCL patients, as well as the recessive V201D mutant) provided insight into the dysfunction relating to human diseases (Bailey SL et al. 2012). The comparative analysis of the exonuclease activity of the dominant mutant TREX1 proteins (homo- and heterodimers generated from wt- and mutant TREX1 monomers) are in agreement with findings of Bailey et al.(Lehtinen DA et al. 2008; Fye JM et al. 2011; Bailey SL et al. 2012).