The soybean vegetative storage proteins VSPα and VSPβ are acid phosphatases active on polyphosphates

The soybean vegetative storage protein genes (vspA, and vspB) are regulated in a complex manner developmentally and in response to external stimuli such as wounding and water deficit. The proteins accumulate to almost one-half the amount of soluble leaf protein when soybean plants are continually depodded and have been identified as storage proteins because of their abundance and pattern of expression in plant tissues. We have shown that purified VSP homodimers (VSPα and VSPβ) and heterodimers (VSPα/β) possess acid phosphatase activity (α = 0.3-0.4 units/mg; β = 2-4 units/mg; α/β = 7-10 units/mg). Specific activities were determined by monitoring o- carboxyphenyl phosphate (0.7 mM) cleavage at pH 5.5 (VSPα) or pH 5.0 (VSPα/β and VSPβ) in 0.15 M sodium acetate buffer at 25 °C. These enzymes are active over a broad pH range, maintaining greater than 40% of maximal activity from pH 4.0 to 6.5 and having maximal activity at pH 5.0-5.5. They are inactivated by sodium fluoride, sodium molybdate, and heating at 70 °C for 10 min. These phosphatases can liberate P(i) from several different substrates, including napthyl acid phosphate, carboxyphenyl phosphate, sugar- phosphates, glyceraldehyde 3-phosphate, dihydroxyacetone phosphate, phosphoenolpyruvate, ATP, ADP, PP(i), and short chain polyphosphates. VSPα/β cleaved phosphoenolpyruvate, ATP, ADP, PP(i), and polyphosphates most efficiently. Apparent K(m) and V(max) values at 25 °C and pH 5.0 were 42 μM and 2.0 μmol/min/mg, 150 μM and 4.2 μmol/min/mg, and 420 μM and 4.1 μmol/min/mg, for tetrapolyphosphate, pyrophosphate, and phosphoenolpyruvate, respectively.