EXPLORING THE ROLE OF THE DUAL-TARGETED 'MAMMALIAN TYPE' PURPLE ACID PHOSPHATASE ATPAP17 IN ARABIDOPSIS THALIANA PHOSPHATE AND ROS METABOLISM

Abstract

Secreted and intracellular purple acid phosphatases (APases) play a central role in the recycling and acquisition of inorganic phosphate (Pi), a limiting macronutrient that is vital for plant growth. In mammals, a single low molecular weight (LMW) secreted purple APase (PAP) isozyme (e.g. HsACP5 in humans) is expressed that exhibits bifunctional phosphatase and peroxidase (PRx) activities to support dual roles in dephosphorylating extracellular bone matrix phosphoproteins or generating reactive oxygen species (ROS) for immune-related pathogen defense. Similarly, AtPAP17 (At3g17790), one of twenty-nine predicted Arabidopsis PAP isozymes, is a LMW (35 kDa) HsACP5 ortholog that exhibits APase and PRx activity. A 35 kDa monomeric PAP was unexpectedly purified from cell wall (CW) extracts of Pi-starved (–Pi) suspension cell cultures of the model plant Arabidopsis thaliana and identified as AtPAP17 (At3g17790). AtPAP17 was de novo synthesized and dual-targeted to the secretome and/or intracellular fraction of –Pi or salt stressed, or senescing leaves of Arabidopsis. Transiently expressed AtPAP17-GFP localized to lytic vacuoles of the Arabidopsis cells. No discernible biochemical or phenotypical changes associated with AtPAP17 loss of function in an atpap17 mutant occurred during Pi deprivation, leaf senescence, or salinity stress. Nevertheless, AtPAP17 is hypothesized to contribute to Pi metabolism owing to its marked upregulation during Pi starvation and leaf senescence, broad APase substrate selectivity and pH-activity profile, and rapid repression and turnover following Pi-resupply to –Pi plants. While AtPAP17 also catalyzed the peroxidation of luminol, which was optimal at pH 9.2, it exhibited a low Vmax and affinity for H2O2 relative to horseradish peroxidase. These results, coupled with absence of a phenotype in the –Pi or salt stressed atpap17 mutant, do not support proposals that AtPAP17’s PRx activity contributes to the detoxification of ROS during stresses that trigger AtPAP17 upregulation.