Single point mutations in various domains of a plant plasma membrane H(+)-ATPase expressed in Saccharomyces cerevisiae increase H(+)-pumping and permit yeast growth at low pH.

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Abstract
In plants, the proton pump‐ATPase (H(+)‐ATPase) of the plasma membrane is encoded by a multigene family. The PMA2 (plasma membrane H(+)‐ATPase) isoform from Nicotiana plumbaginifolia was previously shown to be capable of functionally replacing the yeast H(+)‐ATPase, provided that the external pH was kept above pH 5.5. In this study, we used a positive selection to isolate 19 single point mutations of PMA2 which permit the growth of yeast cells at pH 4.0. Thirteen mutations were restricted to the C‐terminus region, but another six mutations were found in four other regions of the enzyme. Kinetic studies determined on nine mutated PMA2 compared with the wild‐type PMA2 revealed an activated enzyme characterized by an alkaline shift of the optimum pH and a slightly higher specific ATPase activity. However, the most striking difference was a 2‐ to 3‐fold increase of H(+)‐pumping in both reconstituted vesicles and intact cells. These results indicate that point mutations in various domains of the plant H(+)‐ATPase improve the coupling between H(+)‐pumping and ATP hydrolysis, resulting in better growth at low pH. Moreover, the yeast cells expressing the mutated PMA2 showed a marked reduction in the frequency of internal membrane proliferation seen with the strain expressing the wild‐type PMA2, indicating a relationship between H(+)‐ATPase activity and perturbations of the secretory pathway.