THE MAMMALIAN CALCIUM-BINDING PROTEIN, NUCLEOBINDIN (CALNUC), IS A GOLGI RESIDENT PROTEIN

We have identified CALNUC, an EF-hand, Ca2+-binding protein, as a Golg i resident protein. CALNUC corresponds to a previously identified EF-h and/calcium-binding protein known as nucleobindin. CALNUC interacts wi th G alpha(i3) subunits in the yeast two-hybrid system and in GST-CALN UC pull-down assays. Analysis of deletion mutants demonstrated that th e EF-hand and intervening acidic regions are the site of CALNUC's inte raction with G alpha(i3). CALNUC is found in both cytosolic and membra ne fractions. The membrane pool is tightly associated with the luminal surface of Golgi membranes. CALNUC is widely expressed, as it is dete cted by immunofluorescence in the Golgi region of all tissues and cell lines examined. By immunoelectron microscopy, CALNUC is localized to cis-Golgi cisternae and the cis-Golgi network (CGN). CALNUC is the maj or Ca2+-binding protein detected by Ca-45(2+)-binding assay on Golgi f ractions. The properties of CALNUC and its high homology to calreticul in suggest that it may play a key role in calcium homeostasis in the C GN and cis-Golgi cisternae.