Glucose-independent inhibition of yeast plasma-membrane H⁺-ATPase by calmodulin antagonists

Glucose metabolism causes activation of the yeast plasma-membrane H⁺-ATPase. The molecular mechanism of this regulation is not known, but it is probably mediated by phosphorylation of the enzyme. The involvement in this process of several kinases has been suggested but their actual role has not been proved. The physiological role of a calmodulin-dependent protein kinase in glucose-induced activation was investigated by studying the effect of specific calmodulin antagonists on the glucose-induced ATPase kinetic changes in wild-type and two mutant strains affected in the glucose regulation of the enzyme. Preincubation of the cells with calmidazolium or compound 48/80 impeded the increase in ATPase activity by reducing the V_max of the enzyme without modifying the apparent affinity for ATP in the three strains. In one mutant, pma1-T912A, the putative calmodulin-dependent protein kinase-phosphorylatable Thr-912 was eliminated, and in the other, pma1-P536L, H⁺-ATPase was constitutively activated, suggesting that the antagonistic effect was not mediated by a calmodulin-dependent protein kinase and not related to glucose regulation. This was corroborated when the in vitroeffect of the calmodulin antagonists on H⁺-ATPase activity was tested. Purified plasma membranes from glucose-starved or glucose-fermenting cells from both pma1-P890X, another constitutively activated ATPase mutant, and wild-type strains were preincubated with calmidazolium or melittin. In all cases, ATP hydrolysis was inhibited with an IC₅₀ of ≈1 μM. This inhibition was reversed by calmodulin. Analysis of the calmodulin-binding protein pattern in the plasma-membrane fraction eliminates ATPase as the calmodulin target protein. We conclude that H⁺-ATPase inhibition by calmodulin antagonists is mediated by an as yet unidentified calmodulin-dependent membrane protein.