Calcium transport and ATPase activity in a microsomal vesicle fraction from corn roots

An investigation has been made of methods for isolating membrane vesicles from corn (Zea mays L.) roots active in calcium transport and K+-stimulated ATPase. Pretreating and grinding the roots at room temperature with EGTA and fusicoccin increases basal ATPase activity. Improvement in Ca2+ uptake requires isolation of a scaled vesicle fraction by the method of Sze(1980). Sorbitol is superior to sucrose as an osmoticant. The pH optimum for Ca2+ uptake is 7.5. whereas that for associated ATPase activity is 6.5. Calmodulin strongly stimulates Ca2+ uptake in a process little affected by uncouplers and ATPase inhibitors, but blocked by chlorpromazine. Fusicoccin gives less stimulation of Ca2+ uptake which is sensitive to uncouplers, and is dependent upon isolation with fusicoccin present. It appears that the sealed vesicle fraction may possess two Ca2+ transport systems: a calmodulin-activated Ca2+-transporting ATPase, and a Ca2+/H+ antiport coupled through the protonmotive force to a fusicoccin-stimulated H+-ATPase.