Potential regulatory phosphorylation sites in a Medicago truncatula plasma membrane proton pump implicated during early symbiotic signaling in roots

• There are thirteen plasma membrane proton pump genes encoded in the Medicago truncatula genome.
• Brief Nod factor treatment in vivo stimulates the phosphorylation of half a dozen serine and threonine residues in one of the thirteen pump proteins.
• Two of these residues were found to be essential for catalytic activity and growth via heterologous expression and complementation in a yeast proton pump knockout.

In plants and fungi the plasma membrane proton pump generates a large proton-motive force that performs essential functions in many processes, including solute transport and the control of cell elongation. Previous studies in yeast and higher plants have indicated that phosphorylation of an auto-inhibitory domain is involved in regulating pump activity. In this report we examine the Medicago truncatula plasma membrane proton pump gene family, and in particular MtAHA5. Yeast complementation assays with phosphomimetic mutations at six candidate sites support a phosphoregulatory role for two residues, suggesting a molecular model to explain early Nod factor-induced changes in the plasma membrane proton-motive force of legume root cells.