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

Inositol 1,4,5-trisphosphate (IP3) induces a release of Ca2+ from vacu olar membrane vesicles of Saccharomyces cerevisiae. The amount release d is dependent on IP3 concentration (concentration for half maximal ef fect, K(m, apparent) = 0.4 muM). Myo-inositol, and inositol 1,4-bispho sphate 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-diethylam ino)-octyl 3,4,5-trimethoxybenzoate-HCl (TMB-8), which are known to bl ock Ca2+ release from Ca2+ stores in animal cells. IP3-induced release of Ca2+ also occurs when Ca2+ is accumulated by means of an artificia l pH gradient, indicating that the effect of IP3 is not due to an effe ct on the vacuolar H+-ATPase. The IP3-induced Ca2+ release is not acco mpanied 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 DELTAp H by IP3. The present results suggest that IP3 may act as a second mes senger 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.