A phosphothreonine residue at the C-terminal end of the plasma membrane H+-ATPase is protected by fusicoccin-induced 14-3-3 binding

We have isolated the plasma membrane H+-ATPase in a phosphorylated form from spinach (Spinacia oleracea L.) lear tissue incubated with fusicoccin, a fungal toxin that induces irreversible binding of 14-3-3 protein to the C terminus of the H+-ATPase, thus activating H+ pumping. We have identified threonine-948, the second residue from the C-terminal end of the H+-ATPase, as the phosphorylated amino acid. Turnover of the phosphate group of phosphothreonine-948 was inhibited by 14-3-3 binding, suggesting that this residue may form part of a binding motif for 14-3-3. This is the first identification to our knowledge of an in vivo phosphorylation site in the plant plasma membrane H+-ATPase.