THE GOLGI-APPARATUS IS AN INOSITOL 1,4,5-TRISPHOSPHATE-SENSITIVE CA2-PROPERTIES DISTINCT FROM THOSE OF THE ENDOPLASMIC-RETICULUM( STORE, WITH FUNCTIONAL)

In the past few years, intracellular organelles, such as the endoplasm ic reticulum, the nucleus and the mitochondria, have emerged as key de terminants in the generation and transduction of Ca2+ signals of high spatio-temporal complexity. Little is known about the Golgi apparatus, despite the fact that Ca2+ within its lumen controls essential proces ses, such as protein processing and sorting. We report the direct moni toring of the [Ca2+] in the Golgi lumen ([Ca2+](Golgi)) of living HeLa cells, using a specifically targeted Ca2+-sensitive photoprotein. Wit h this probe, we show that, in resting cells, [Ca2+](Golgi) is similar to 0.3 mM and that Ca2+ accumulation by the Golgi has properties dist inct from those of the endoplasmic reticulum (as inferred by the sensi tivity to specific inhibitors). Upon stimulation with histamine, an ag onist coupled to the generation of inositol 1,4,5-trisphosphate (IP3), a large, rapid decrease in [Ca2+](Golgi) is observed. The Golgi appar atus can thus be regarded as a bona fide IP3-sensitive intracellular C a2+ store, a notion with major implications for the control of organel le function, as well as for the generation of local cytosolic Ca2+ sig nals.