Regulation of H+Pumping by Plant Plasmalemma under Osmotic Stress: The Role of 14-3-3 Proteins

All higher plants have high-specific sites for binding fusicoccin (FCBS), a metabolite of the fungus Fusicoccum amygdaliDel. These sites are localized on the plasmalemma and produced by the association of the dimers of 14-3-3 proteins with the C-terminal autoinhibitory domain of H+-ATPase. Considering the fusicoccin binding to the plasmalemma as an index characterizing the formation of this complex, we studied the influence of osmotic stress on the interaction between 14-3-3 proteins and H+-ATPase in the suspension-cultured sugar beet cells and protoplasts obtained from them. An increase in the osmolarity of the extracellular medium up to 0.3 Osm was shown to enhance proton efflux from the cells by several times. The number of FCBS in isolated plasma membranes increased in parallel, whereas 14-3-3 proteins accumulated in this membrane to a lesser degree. The amount of H+-ATPase molecules did not change, and the ATP-hydrolase activity changed insignificantly. The data obtained indicate that osmotic stress affects H+-ATPase pumping in the plasmalemma through its influence on the coupling between H+-transport and ATP hydrolysis; 14-3-3 proteins are involved in this coupling. The interaction between the plasmalemma and the cell wall is suggested to be very important in this process.