INOSITOL 1,4,5-TRISPHOSPHATE RELEASES CA2+ FROM VACUOLAR MEMBRANE-VESICLES OF SACCHAROMYCES-CEREVISIAE

Inositol 1,4,5-trisphosphate (IP3) induces a release of Ca2+ from vacuolar membrane vesicles of Saccharomyces cerevisiae. The amount released is dependent on IP3 concentration (concentration for half maximal effect, K(m, apparent) = 0.4 muM). Myo-inositol, and inositol 1,4-bisphosphate up to 50 muM have no effect on Ca2+ levels in the vesicles. The IP3-induced Ca2+ release is blocked by dantrolene and 8-(NN-diethylamino)-octyl 3,4,5-trimethoxybenzoate-HCl (TMB-8), which are known to block Ca2+ release from Ca2+ stores in animal cells. IP3-induced release of Ca2+ also occurs when Ca2+ is accumulated by means of an artificial pH gradient, indicating that the effect of IP3 is not due to an effect on the vacuolar H+-ATPase. The IP3-induced Ca2+ release is not accompanied by a change in the pH gradient, which indicates that it is not due to a reversal of the Ca2+/nH+ antiport or to a decrease in DELTApH by IP3. The present results suggest that IP3 may act as a second messenger in the mobilization of Ca2+ in yeast cells. As in plant cells, the vacuolar membrane of yeast seems to contain a Ca2+ channel, which can be opened by IP3. In this respect the vacuole could function as an IP3-regulated intracellular Ca2+ store, equivalent to the endoplasmic- and sarcoplasmic reticulum in animal cells, and play a role in Ca2+-dependent signal transduction in yeast cells.