CERCOSPORA-BETICOLA TOXINS .17. THE ROLE OF THE BETICOLIN MG2+ COMPLEXES IN THEIR BIOLOGICAL-ACTIVITY STUDY OF PLASMA-MEMBRANE H+-ATPASE, VACUOLAR H+-PPASE, ALKALINE AND ACID-PHOSPHATASES/

Beticolin-1 and beticolin-2, yellow toxins produced by the phytopathog enic fungus Cercospora beticola, inhibit the plasma membrane H+-ATPase . Firstly, since beticolins are able to form complexes with Mg2+, the role of the beticolin/Mg2+ complexes in the inhibition of the plasma m embrane proton pump has been investigated. Calculations indicate that beticolins could exist under several forms, in the H+-ATPase assay mix ture, both free or complexed with Mg2+, However, the percentage inhibi tion of the H+-ATPnse activity is correlated to the concentration of o ne single form of beticolin, the dimeric neutral complex Mg2H2B2, whic h appears to be the active form involved in the Kc-ATPase inhibition, Secondly, since previous data suggested that beticolins could also be active against other Mg2+-dependent enzymes, we rested beticolin-1 on the vacuolar H+-PPase, which requires Mg2+ as co-substrate, and on the alkaline and acid phosphatases, which do not use Mg2+ as co-substrate . Only vacuolar H+-PPase is sensitive to beticolin-1, which suggests t hat beticolins are specific to enzymes that use a complex of Mg2+ as t he substrate. The same Mg2H2B2 complex which is responsible of the pla sma membrane H+-ATPase inhibition appears to be also involved in the i nhibition of the vacuolar H+-PPase.