Heterogeneity of Ca2+ handling among and within Golgi compartments

The Golgi apparatus (GA) is a dynamic intracellular Ca21 store endowed with complex Ca21 homeostatic mechanisms in part distinct from those of the endoplasmic reticulum (ER). We describe the generation of a novel fluorescent Ca21 probe selectively targeted to the medialGolgi. We demonstrate that in the medial-Golgi: (i) Ca21 accumulation takes advantage of two distinct pumps, the sarco/endoplasmic reticulum Ca21 ATPase and the secretory pathway Ca21 ATPase1; (ii) activation of IP3 or ryanodine receptors causes Ca21 release, while no functional two-pore channel was found; (iii) luminal Ca21 concentration appears higher than that of the trans-Golgi, but lower than that of the ER, suggesting the existence of a cis- to trans-Golgi Ca21 concentration gradient. Thus, the GA represents a Ca21 store of high complexitywhere,despite the continuousflowofmembranes and luminalcontents, eachsub-compartmentmaintainsits Ca21 identity with specific Ca21 homeostatic characteristics. The functional role of such micro-heterogeneity in GA Ca21 handling is discussed.