InterPro domain: IPR010028

This entry represents a family of acid phosphatase [ 1 , 2 ] from plants which are closely related to the class B non-specific acid phosphatase OlpA ( IPR006423 , which is believed to be a 5'-nucleotide phosphatase) and somewhat more distantly to another class B phosphatase, AphA ( IPR010025 ). Together these three clades define a subfamily of Acid phosphatase (Class B), which corresponds to the IIIB subfamily of the haloacid dehalogenase (HAD) superfamily of aspartate nucleophile hydrolases. It has been reported that the best substrates were purine 5'-nucleoside phosphates [ 3 ]. This is in concordance with the assignment of the Haemophilus influenzae hel protein (from IPR006423 ) as a 5'-nucleotidase, however there is presently no other evidence to support this specific function for this family of plant phosphatases. Many genes from this family have been annotated as vegetative storage proteins (VSPs) due to their close homology with these earlier-characterised gene products which are highly expressed in leaves. There are significant differences however, including expression levels and distribution [ 3 ]. The most important difference is the lack in authentic VSPs of the nucleophilic aspartate residue, which is instead replaced by serine, glycine or asparagine. Thus these proteins can not be expected to be active phosphatases. This issue was confused by the publication in 1992 of an article claiming activity for the Glycine max (Soybean) VSP [ 4 ]. In 1994 this assertion was refuted by the separation of the activity from the VSP. This entry explicitly excludes the VSPs which lack the nucleophilic aspartate. The possibility exists, however, that some members of this family may, while containing all of the conserved HAD-superfamily catalytic residues, lack activity and have a function related to the function of the VSPs rather than the acid phosphatases.

1. Purification and characterization of a soybean root nodule phosphatase expressed in Pichia pastoris. Protein Expr. Purif. 14, 125-30
2. Soybean root nodule acid phosphatase. Plant Physiol. 114, 597-604
3. Arabidopsis thaliana vegetative storage protein (VSP) genes: gene organization and tissue-specific expression. Plant Mol. Biol. 38, 565-76
4. The soybean vegetative storage proteins VSP alpha and VSP beta are acid phosphatases active on polyphosphates. J. Biol. Chem. 267, 15958-64