Cercospora beticola toxins. Part XVII. The role of the beticolin/Mg 2+ complexes in their biological activity Study of plasma membrane H +-ATPase, vacuolar H +-PPase, alkaline and acid phosphatases

Abstract

Beticolin-1 and beticolin-2, yellow toxins produced by the phytopathogenic fungus Cercospora beticola, inhibit the plasma membrane H +-ATPase. Firstly, since beticolins are able to form complexes with Mg 2+, the role of the beticolin/Mg 2+ complexes in the inhibition of the plasma membrane proton pump has been investigated. Calculations indicate that beticolins could exist under several forms, in the H +-ATPase assay mixture, both free or complexed with Mg 2+. However, the percentage inhibition of the H +-ATPase activity is correlated to the concentration of one single form of beticolin, the dimeric neutral complex Mg 2H 2B 2, which appears to be the active form involved in the H +-ATPase inhibition. Secondly, since previous data suggested that beticolins could also be active against other Mg 2+-dependent enzymes, we tested beticolin-1 on the vacoular H +-PPase, which requires Mg 2+ as co-substrate, and on the alkaline and acid phosphatases, which do not use Mg 2+ as co-substrate. Only vacuolar H +-PPase is sensitive to beticolin-1, which suggests that beticolins are specific to enzymes that use a complex of Mg 2+ as the substrate. The same Mg 2H 2B 2 complex which is responsible of the plasma membrane H +-ATPase inhibition appears to be also involved in the inhibition of the vacuolar H +-PPase.