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The plasma-membrane H+-pump in guard cells generates the driving force for the rapid ion fluxes required for stomatal opening. Since our electrophysiological studies revealed a two fold higher pump-current density in guard cells than in mesophyll cells of Viciafaba L. we elucidated the biochemical properties of this proton-translocating ATPase in plasma-membrane vesicles isolated from both cell types. The capability of the H+-ATPase to create an H + gradient is maintained in plasma-membrane vesicles derived from purified guard cells via blender maceration, high-pressure homogenization and polymer separation. The H+-pumping activity of these vesicles coincides with the presence of two polypeptides of approx. 100 and 92 kDa which are recognized by a monoclonal antibody raised against the plasma-membrane H+-ATPase from Zea mays L. coleoptiles. Comparison of H +-pumping activities of isolated membranes revealed an approximately two fold higher activity in guard cells than in mesophyll cells with respect to the total membrane protein content. Furthermore, we demonstrated by western blotting that the difference in pump activities resulted from a higher abundance of the electroenzyme per unit membrane protein in guard-cell plasma membranes. We suggest that the high H+-pump capacity is necessary to enable guard cells to respond to sudden changes in the environment by a change in stomatal aperture.Abbreviations: PM = plasma membrane; SDS-PAGE = sodium dodecyl sulfate-polyacrylamide gel electrophoresis

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