| Acid phosphatases (APs) function in the production and recycling of Pi, a crucial but limiting macronutrient for plant growth and metabolism. Quantification of immunoblots revealed that the ∼4-fold increase in AP activity that occurred following 7-d of Pi deprivation of Arabidopsis thaliana suspension cells was paralleled by a similar increase in the amount of an anti-(tomato purple AP (PAP))-IgG immunoreactive 55-kDa polypeptide. An AP from Pi deficient (-Pi) Arabidopsis cells was purified 957-fold to homogeneity and a final phosphoenolpyruvate-hydrolyzing specific activity of 421 units/mg protein. The final AP preparation was pink in colour and insensitive to tartrate inhibition, indicating that it is a PAP. BLAST analysis of its N-terminal sequence demonstrated that it is encoded by 1 of 29 putative Arabidopsis PAP genes, specifically AtPAP26, and that a 30 amino acid signal peptide is cleaved from the preprotein following its translocation into the vacuole. AtPAP26 exists as a ∼100 kDa homodimer of 55-kDa glycosylated subunits, and displayed a pH-activity optimum of 5.6, activation by Mg2+, but potent inhibition by Zn2+, Fe2+, molybdate, and phosphate. Furthermore, the AP exhibited significant peroxidase activity. Immunoblotting of roots and shoots from Pi sufficient (+Pi) vs. -Pi Arabidopsis seedlings indicated that this PAP may be root specific under short-term Pi starvation. This is the first Pi starvation inducible intracellular Arabidopsis PAP that has been purified and characterized. It is hypothesized that this PAP plays a pivotal role in Pi remobilization and scavenging from intracellular P-monoesters in -Pi Arabidopsis. |
