ACTIVITY OF THE PLASMA-MEMBRANE H-ATPASE IS A KEY PHYSIOLOGICAL DETERMINANT OF THERMOTOLERANCE IN SACCHAROMYCES-CEREVISIAE()

The role of membrane integrity and the membrane ATPase in the mechanis m of thermotolerance in Saccharomyces cerevisiae was investigated. The resistance to lethal heat of a mutant strain with reduced expression of the membrane ATPase was significantly less than that of the wild-ty pe parent. However, prior exposure to sub-lethal temperatures resulted in the induction of similar levels of thermotolerance in the mutant c ompared to the parent strain, suggesting that the mechanism of sub-let hal heat-induced thermotolerance is independent of ATPase activity. Su pporting this, exposure to sub-lethal heat stress did not result in in creased levels of glucose-induced acid efflux at lethal temperatures a nd there was little correlation between levels of acid efflux and leve ls of heat resistance. ATPase activity in crude membrane preparations from sub-lethally heat-stressed cells was similar to that in preparati ons from unstressed cells. Study of net acid flux during heating revea led that pre-stressed cells were able to protect the proton gradient f or longer. This may confer an 'advantage' to these cells that results in increased thermotolerance. This was supported by the observation th at prior exposure to sub-lethal heat resulted in a transient protectio n against the large increase in membrane permeability that occurs at l ethal temperatures. However. no protection against the large drop in i ntracellular pH was detected. Sub-lethal heat-induced protection of me mbrane integrity also occurred to the same extent in the reduced-expre ssion membrane ATPase mutant further implying that the mechanism of in duced thermotolerance is independent of ATPase activity. To conclude, although the membrane ATPase is essential for basal heat resistance, t hermotolerance induced by prior exposure to stress is largely conferre d by a mechanism that is independent of the enzyme.