A single amino acid substitution in soybean VSPα increases its acid phosphatase activity nearly 20-fold

Authors

Oranuch Leelapon, University of Nebraska-Lincoln
Gautam Sarath, University of Nebraska-LincolnFollow
Paul E. Staswick, University of Nebraska-LincolnFollow

Date of this Version

9-30-2004

Comments

Published in Planta (2004) 219: 1071–1079.

Abstract

Soybean [Glycine max (L.) Merr.] contains two proteins called vegetative storage proteins (VSPs) that function as temporary storage reserves, but are also closely related to plant acid phosphatases of the haloacid dehalogenase (HAD) superfamily. This study examined the biochemical basis for the relatively low catalytic activity previously reported for these VSPs. The specific activity of purified recombinant VSPα on GMP was about 40-fold lower than for a related soybean root nodule acid phosphatase (APase), which had a specific activity of 845 U mg^-1 protein. Conversion of Ser¹⁰⁶ to Asp increased VSPα activity about 20-fold. This Asp residue is present in nodule APase and is a highly conserved nucleophile in the HAD superfamily. Related VSPs from cultivated soybean and from three wild perennial soybeans, as well as a pod storage protein (PSP) from Phaseolus vulgaris L. all lack the catalytic Asp, suggesting they too are catalytically inefficient. Phylogenetic analysis showed the VSPs and PSP are more closely related to each other than to 21 other VSPlike proteins from several plant species, all of which have the nucleophilic Asp. This study suggests that loss of catalytic activity may be a requirement for the VSPs and PSP to function as storage proteins in legumes.