Regulation and pH-dependent expression of a bilaterally truncated yeast plasma membrane H+-ATPase

Constitutive, chromosomal expression of yeast pma1 deletion alleles in Saccharomyces cerevisiae yielded functional, truncated forms of the plasma membrane H+-ATPase which were independently capable of supporting wild type yeast growth rates. Deletion of 27 amino-terminal residues affected neither the enzyme's activity nor its responsiveness to changes in glucose metabolism. By contrast, removal of 18 carboxy-terminal amino acids produced an enzyme with a Vmax that was relatively insensitive to glucose-dependent metabolic status and with a Km that was significantly lower than that of the wild type enzyme. These effects were exaggerated when the amino- and carboxy-terminal deletions were combined in a bilaterally truncated H+-ATPase, suggesting that the amino terminus may have a subtle role in modulating ATPase activity. In pma1DeltaDelta cells cultured at pH 6, plasma membrane H+-ATPase levels were much lower than those in cells expressing a wild type ATPase. Increased expression levels could be achieved by growing the pma1DeltaDelta mutant at pH 3, a result that was at least partially due to a sustained, elevated transcription of pma1DeltaDelta mRNA. Our observations suggest that intracellular proton balance can be maintained by regulation of the activity and/or quantity of H+-ATPase in the plasma membrane.