Biochemical and cellular characteristics of the 3' -> 5' exonuclease TREX2

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Abstract
TREX2 is an autonomous nonprocessive 3′ → 5′ exonuclease, suggesting that it maintains genome integrity. To investigate TREX2's biochemical and cellular properties, we show that endogenous TREX2 is expressed widely in mouse tissues and human cell lines. Unexpectedly, endogenous human TREX2 is predominantly expressed as a 30-kDa protein (not 26 kDa, as previously believed), which is likely encoded by longer isoforms (TREX2L1 and/or TREX2L2) that possess similar capacity for self-association, DNA binding and catalytic activity. Site-directed mutagenesis analysis shows that the three functional activities of TREX2 are distinct, yet integrated. Mutation of amino acids putatively important for homodimerization significantly impairs both DNA binding and exonuclease activity, while mutation of amino acids (except R163) in the DNA binding and exonuclease domains affects their corresponding activities. Interestingly, however, DNA-binding domain mutations do not impact catalytic activity, while exonuclease domain mutations diminish DNA binding. To understand TREX2 cellular properties, we find endogenous TREX2 is down regulated during G2/M and nuclear TREX2 displays a punctate staining pattern. Furthermore, TREX2 knockdown reduces cell proliferation. Taken together, our results suggest that TREX2 plays an important function during DNA metabolism and cellular proliferation.