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Published on September 8, 2006; 10.1104/pp.106.087171

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Received July 21, 2006
Accepted September 6, 2006

Biochemical and Molecular Characterization of AtPAP26, a Vacuolar Purple Acid Phosphatase Upregulated in Phosphate-Deprived Arabidopsis Suspension Cells and Seedlings

Vasko Veljanovski , Barbara Vanderbeld , Vicki L. Knowles , Wayne A. Snedden , and William C. Plaxton *

Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6
Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6, Department of Biochemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6

* Corresponding author; email: plaxton{at}biology.queensu.ca.

A vacuolar acid phosphatase (APase) that accumulates during Pi-starvation of Arabidopsis thaliana suspension cells was purified to homogeneity. The final preparation is a purple APase (PAP) as it exhibited a pink color in solution (Amax = 520 nm). It exists as a 100-kD homodimer composed of 55-kD glycosylated subunits that cross-reacted with an anti-(tomato intracellular PAP)-IgG. BLAST analysis of its 23 amino acid N-terminal sequence revealed that this PAP is encoded by At5g34850 (AtPAP26; 1 of 29 PAP genes in Arabidopsis), and that a 30 amino acid signal peptide is cleaved from the AtPAP26 preprotein during its translocation into the vacuole. AtPAP26 displays much stronger sequence similarity to orthologs from other plants than to other Arabidopsis PAPs. AtPAP26 exhibited optimal activity at pH 5.6 and broad substrate selectivity. The 5-fold increase in APase activity that occurred in Pi-deprived cells was paralleled by a: (i) similar increase in the amount of a 55-kD anti-(tomato PAP or AtPAP26)-IgG immunoreactive polypeptide, and (ii) >30-fold reduction in intracellular free Pi concentration. Semi-quantitative RT-PCR indicated that Pi-sufficient, Pi-starved, and Pi-resupplied cells contain similar amounts of AtPAP26 transcripts. Thus, transcriptional controls appear to exert little influence on AtPAP26 levels, relative to translational and/or proteolytic controls. APase activity and AtPAP26 protein levels were also upregulated in shoots and roots of Pi-deprived Arabidopsis seedlings. We hypothesize that AtPAP26 recycles Pi from intracellular P-metabolites in Pi-starved Arabidopsis. As AtPAP26 also exhibited alkaline peroxidase activity, a potential additional role in the metabolism of reactive oxygen species is discussed.




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