| Abstract: | PMA1 expression, plasma membrane H(+)-ATPase enzyme kinetics, and the distribution of the ATPase have been studied in carbon-starved Candida albicans induced with glucose for yeast growth at pH 4.5 and for germ tube formation at pH 6.7. PMA1 expression parallels expression of the constitutive ADE2 gene, increasing up to sixfold during yeast growth and twofold during germ tube formation. Starved cells contain about half the concentration of plasma membrane ATPase of growing cells. The amount of plasma membrane ATPase is normalized prior to either budding or germ tube emergence by the insertion of additional ATPase molecules, while ATPase antigen appears uniformly distributed over the entire plasma membrane surface during both growth phases. Glucose addition rapidly activates the ATPase twofold regardless of the pH of induction. The turnover of substrate molecules per second by the enzyme in membranes from budding cells quickly declines, but the enzyme from germ tube-forming cells maintains its turnover of substrate molecules per second and a higher affinity for Mg-ATP. The plasma membrane ATPase of C. albicans is therefore regulated at several levels; by glucose metabolism/starvation-related factors acting on gene expression, by signals generated through glucose metabolism/starvation which are thought to covalently modify the carboxyl-terminal domain of the enzyme, and possibly by additional signals which may be specific to germ tube formation. The extended period of intracellular alkalinization associated with germ tube formation may result from regulation of proton-pumping ATPase activity coupled with higher ratios of cell surface to effective cytosolic volume. |
