Biochemical and Molecular Characterization of AtPAP12 and AtPAP26: the Predominant Purple Acid Phosphatase Isozymes Secreted by Phosphate-starved Arabidopsis Thaliana

Plant purple acid phosphatases (PAPs) belong to a large multigene family whose specific functions in Pi metabolism are poorly understood. Two PAP isozymes secreted by Pi-deficient (-Pi) Arabidopsis thaliana were purified from culture filtrates of -Pi suspension cells. They correspond to an AtPAP12 (At2g27190) homodimer and AtPAP26 (At5g34850) monomer composed of glycosylated 60 and 55 kDa subunit(s), respectively. Each PAP exhibited broad pH activity profiles centred at pH 5.6, and overlapping substrate specificities. Concanavalin-A chromatography resolved a pair of secreted AtPAP26 glycoforms. AtPAP26 is dual targeted during Pi stress because it is also the principal intracellular (vacuolar) PAP up-regulated by -Pi Arabidopsis. Differential glycosylation appears to influence the subcellular targeting and substrate selectivity of AtPAP26. The significant increase in secreted acid phosphatase activity of -Pi seedlings was correlated with the appearance of immunoreactive AtPAP12 and AtPAP26 polypeptides. Analysis of atpap12 and atpap26 T-DNA mutants verified that AtPAP12 and AtPAP26 account for most of the secreted acid phosphatase activity of -Pi wild-type seedlings. Semi-quantitative RT-PCR confirmed that transcriptional controls exert little influence on the up-regulation of AtPAP26 during Pi stress, whereas AtPAP12 transcripts correlate well with relative levels of secreted AtPAP12 polypeptides. We hypothesize that AtPAP12 and AtPAP26 facilitate Pi scavenging from soil-localized organophosphates during nutritional Pi deprivation.