Biochemical and molecular characterization of AtPAP17: a dual-localized, low molecular weight Arabidopsis purple acid phosphatase upregulated during phosphate deprivation, senescence, and oxidative stress.

A 35 kDa monomeric purple acid phosphatase (APase) was purified from cell wall extracts of Pi starved (-Pi) Arabidopsis thaliana suspension cells and identified as AtPAP17 (At3g17790) by mass spectrometry and N-terminal microsequencing. AtPAP17 was de novo synthesized and dual-localized to the secretome and/or intracellular fraction of -Pi or salt stressed plants, or senescing leaves. Transiently expressed AtPAP17-GFP localized to lytic vacuoles of the Arabidopsis suspension cells. No significant biochemical or phenotypical changes associated with AtPAP17 loss-of-function were observed in an atpap17 mutant 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 hydrogen peroxide relative to horseradish peroxidase. These results, coupled with absence of a phenotype in the salt stressed or -Pi atpap17 mutant, do not support proposals that AtPAP17's peroxidase activity contributes to the detoxification of reactive oxygen species during stresses that trigger AtPAP17 upregulation.