GLUCOSE-INDUCED ACTIVATION OF THE PLASMA-MEMBRANE H+-ATPASE IN FUSARIUM-OXYSPORUM

Addition of glucose and other sugars to derepressed cells of the fungus Fusarium oxysporum var. lini triggered activation of the plasma membrane H+-ATPase within 5 min. Glucose was the best activator while galactose and lactose had a lesser effect. The activation was not prevented by previous addition of cycloheximide and it was fully reversible when the glucose was removed. The activation process in vivo also caused changes in the kinetic properties of the enzyme. The non-activated enzyme had an apparent K(m) of about 3.2 mM for ATP whereas the activated enzyme showed an apparent K(m) of 0.26 mM. In addition, the pH optimum of the H+-ATPase changed from 6.0 to 7.5 upon activation. The activated enzyme was more sensitive to inhibition by vanadate. When F. oxysporum was cultivated in media containing glucose as the major carbon source, enhanced H+-ATPase activity was largely confined to the period corresponding to the lag phase, i.e. just before the start of acidification of the medium. This suggests that the activation process might play a role in the onset of extracellular acidification. Addition of glucose to F. oxysporum var. lini cells also caused an increase in the cAMP level. No reliable increase could be demonstrated for the other sugars. Addition of proton ionophores such as DNP and CCCP at pH 5.0 caused both a large increase in the intracellular level of cAMP and in the activity of the plasma membrane H+-ATPase. Inhibition of the DNP-induced increase in the cAMP level by acridine orange also resulted in inhibition of the activation of plasma membrane H+-ATPase. These results suggest a possible causal relationship between the activity of F. oxysporum var. lini plasma membrane H+-ATPase and the intracellular level of cAMP.