Differential response of vacuolar proton pumps to osmotica

The vacuole is a fundamental and dominant organelle and occupies a large part of the total cell volume in most mature plant cells. The higher-plant vacuole contains two types of proton-translocating pumps, H⁺-ATPase (EC 3.6.1.3) and H⁺-pyrophosphatase (EC 3.6.1.1), residing on the same membrane. These two enzymes generate roughly equal proton gradients across the vacuolar membrane for the secondary transport of ions and metabolites. However, the pumps respond differentially to stress in order to maintain critical functions of the vacuole. In this work, tonoplasts from etiolated mung bean seedlings (Vigna radiata L.) were used to investigate the function of these two enzymes under high osmotic pressure. At high concentrations of sucrose or sorbitol, the light scattering and volume of isolated vesicles were progressively changed. Concomitantly, enzymatic activities, proton translocation, and coupling efficiencies of these two proton-pumping enzymes were inhibited to various extents under high osmotic pressure. No significant change in enzymatic activities of purified vacuolar H⁺-PPase and H⁺-ATPase under similar conditions was observed. We thus believe that the membrane structure is an important determinant for proper function of proton pumping systems of plant vacuoles. Furthermore, kinetic analysis shows different variation in apparent V_max but not in K_M values of vacuolar H⁺-PPase and H⁺-ATPase at high osmolarity of sucrose and sorbitol, respectively, suggesting probable alterations in substrate hydrolysis reactions but not substrate-binding affinity of the enzymes. A working model is proposed to interpret supplemental roles of vacuolar H⁺-PPase and H⁺-ATPase to maintain appropriate functions of plant tonoplasts.