Towards a New Model for the TREX1 Exonuclease

The TREX1 3 [->] 5 exonuclease degrades DNA in vivo to prevent chronic immune activation through the cGAS-STING pathway. TREX1 degrades ss- and dsDNA containing a free 3-hydroxyl, but the precise nature of immune-activating DNA remains an open question. The TREX1 homodimer structure is critical for exonuclease activity with amino acids from one protomer acting across the dimer interface contributing to catalysis in the opposing protomer. The unique TREX1 obligate homodimer structure suggests an intricate connection between the TREX1 protomers that has yet to be explained. We used biochemical assays, molecular dynamics simulations, and kinetic modeling to determine relative TREX1 affinities for ss- and dsDNA and to interrogate inter-protomer communication within the TREX1 homodimer. These new findings indicate that TREX1 is a semi-processive exonuclease with at least a 20-fold greater affinity for dsDNA than for ssDNA. Furthermore, we find extensively correlated dynamics between TREX1 protomers revealing newly identified substrate interactions in the TREX1 enzyme. These data indicate that TREX1 has evolved as a semi-processive exonuclease with a likely in vivo function to degrade dsDNA, where the TREX1 homodimer structure facilitates a mechanism for efficient binding and catabolism of dsDNA. These studies identify previously unrecognized regions of the TREX1 enzyme involved in DNA interactions, and our findings contribute to an emerging model of TREX1 exonuclease activity.