Triphosphate Tunnel Metalloenzyme Function in Senescence Highlights a Biological Diversification of This Protein Superfamily.

The triphosphate tunnel metalloenzyme (TTM) superfamily comprises a group of enzymes that hydrolyze organophosphate substrates. They exist in all domains of life, yet the biological role of most family members is unclear. Arabidopsis () encodes three genes. We have previously reported that AtTTM2 displays pyrophosphatase activity and is involved in pathogen resistance. Here, we report the biochemical activity and biological function of and diversification of the biological roles between and Biochemical analyses revealed that AtTTM1 displays pyrophosphatase activity similar to AtTTM2, making them the only TTMs characterized so far to act on a diphosphate substrate. However, knockout mutant analysis showed that is not involved in pathogen resistance but rather in leaf senescence. is transcriptionally up-regulated during leaf senescence, and knockout mutants of exhibit delayed dark-induced and natural senescence. The double mutant of and did not show synergistic effects, further indicating the diversification of their biological function. However, promoter swap analyses revealed that they functionally can complement each other, and confocal microscopy revealed that both proteins are tail-anchored proteins that localize to the mitochondrial outer membrane. Additionally, transient overexpression of either gene in induced senescence-like cell death upon dark treatment. Taken together, we show that two TTMs display the same biochemical properties but distinct biological functions that are governed by their transcriptional regulation. Moreover, this work reveals a possible connection of immunity-related programmed cell death and senescence through novel mitochondrial tail-anchored proteins.