Does plasma membrane H+‐ATPase activation by fusicoccin involve protein kinase?

Abstract

Sugar beet (Beta vulgaris L.) root suspension‐cultured cells were converted to protoplasts which responded to fusicoccin (FC) by a rise in cytoplasmic pH (pHcyt) averaging 0.25 units in the fluorimetric assay. This effect was blocked by erythrosin B, a specific inhibitor of the plasma membrane H+‐ATPase. A protein kinase inhibitor, staurosporine also caused cytosolic alkalinization that was sensitive to H+‐ATPase inhibitors. Most strikingly, the effect of staurosporine was suppressed by fusicoccin and vice versa. Addition of okadaic acid, entailing overall protein phosphorylation, also led to H+‐ATPase activation, whereupon fusicoccin lost its effect on proton transport. In parallel, kinetic and inhibitor analyses demonstrated that FC binding to the protoplast plasma membrane involved two sites with dissociation constants of 1 nM and 0.2 μM and was indifferent to phosphorylation and dephosphorylation inhibitors. Thus, it could be concluded that (1) the effect of FC on cytoplasmic pH probably depends on the phosphorylation state of plasma membrane proteins and may have either sign; (2) the activation of H+‐ATPase by FC most likely proceeds directly through conformational receptor‐enzyme interaction.