The plasma membrane H(+)-ATPase of fungi. A candidate drug target?

The fungal plasma membrane H(+)-ATPase possesses important attributes that make it desirable as a target for antifungal drug discovery. First, the enzyme is essential to fungal cell physiology, being required for the formation of a large electrochemical proton gradient and the maintenance of intracellular pH. While complete inhibition of the proton pump will certainly be lethal, partial inhibition can also be lethal depending on the environment of the cell (gastrointestinal tract, etc.). Thus, an effective antagonist of the proton pump will be fungicidal, which is an important attribute for a drug being developed to treat opportunistic infections in the severely immunocompromised. Secondly, the well-characterized biochemistry and genetics of the H(+)-ATPase (encoded by the PMA1 gene) facilitate detailed analysis of interaction of lead or model compounds with the enzyme. Studies with omeprazole, which is not suitable as an antifungal but can be used under selective conditions to target H(+)-ATPase, indicate that the enzyme can be inhibited from its extramembrane surface. Detailed genetic analysis suggests that modification of amino acids in transmembrane segments 1 and 2 can either enhance or diminish the omeprazole sensitivity of the H(+)-ATPase, depending on the nature and location of the amino acid substitution. This region in mammalian P-type enzymes has been implicated in the interaction of cardiac glycosides and reversible gastric pump inhibitors. Our results suggest that this region in the H(+)-ATPase may be valuable as a potential interaction domain for antifungal agents. Finally, a number of primary and secondary screens are available to identify compounds that are targeted to the H(+)-ATPase and affect one or more functional properties. These screens assess enzyme functionality in the cell as well as in vitro and can be used in 96-well microplate format to facilitate high through-put screening. These screens have already yielded promising H(+)-ATPase-directed antagonists. In conclusion, the plasma membrane H(+)-ATPase is a highly desirable target for the development of novel antifungal therapeutics.