Subunit interaction of vacuolar H+-pyrophosphatase as determined by high hydrostatic pressure

Vacuolar H⁺-pyrophosphatase (H⁺-PPase) from etiolated hypocotyls of mung bean (Vigna radiata L.) is a homodimer with a molecular mass of 145 kDa. The vacuolar H⁺-PPase was subjected to high hydrostatic pressure to investigate its structure and function. The inhibition of H⁺-PPase activity by high hydrostatic pressure has a pressure-, time- and protein-concentration-dependent manner. The V_max value of vacuolar H⁺-PPase was dramatically decreased by pressurization from 293.9 to 70.2 µmol of PP_i (pyrophosphate) consumed/h per mg of protein, while the K_m value decreased from 0.35 to 0.08 mM, implying that the pressure treatment increased the affinity of PP_i to vacuolar H⁺-PPase but decreased its hydrolysis. The physiological substrate and its analogues enhance high pressure inhibition of vacuolar H⁺-PPase. The HPLC profile reveals high pressure treatment of H⁺-PPase provokes the subunit dissociation from an active into inactive form. High hydrostatic pressure also induces the conformational change of vacuolar H⁺-PPase as determined by spectroscopic techniques. Our results indicate the importance of protein–protein interaction for this novel proton-translocating enzyme. Working models are proposed to interpret the pressure inactivation of vacuolar H⁺-PPase. We also suggest that association of identical subunits of vacuolar H⁺-PPase is not random but proceeds in a specific manner.