Regulation of intra-Golgi membrane transport by calcium

Calcium cations play a critical role in regulating vesicular transport between different intracellular membrane-bound compartments. The role of calcium in transport between the Golgi cisternae, however, remains unclear. Using a well characterized cell-free intra-Golgi transport assay, we now show that changes in free Ca2+ concentration in the physiological range regulate this transport process. The calcium-chelating agent 1,2-bis(2-aminophenoxy)ethane-N,N,N,N'-tetraacetic acid blocked transport with an IC50 of approximately 0.8 mM. The effect of 1,2-bis(2- aminophenoxy)ethane-N,N,N',N'-tetraacetic acid was reversible by addition of fresh cytosol and was irreversible when performed in the presence of a Ca2+ ionophore that depletes calcium from lumenal stores, We demonstrate here that intra-Golgi transport is stimulated by low Ca2+ concentrations (20-100 nM) but is inhibited by higher concentrations (above 100 nM). Further, we show that calmodulin antagonists specifically block intra-Golgi transport, implying a role for calmodulin in mediating the effect of calcium. Our results suggest that Ca2+ efflux from intracellular pools may play an essential role in regulating intra-Golgi transport.