Calnuc, an EF-hand Ca(2+) binding protein, specifically interacts with the C-terminal alpha5-helix of G(alpha)i3

Calnuc (nucleobindin) was previously shown to be present both in the cytosol and in the Golgi and to be the major Golgi Ca(2+) binding protein. In this study we verified the existence of the cytosolic pool of calnuc and investigated its interaction with G(alpha)i3. Cytosolic calnuc was released by mild digitonin permeabilization. In pulse-chase experiments, the two pools of calnuc had different mobilities, suggesting different posttranslational modifications. That calnuc interacts with G(alpha)i3 in vivo was verified by the finding that G(alpha)i3 could be crosslinked intracellularly to calnuc and co-immunoprecipitated from NIH 3T3 cells stably overexpressing either activated (Q204L) or inactivated (G203A) G(alpha)i3. Binding was Ca(2+) and Mg(2+)-dependent. Calnuc and G(alpha)i3-GFP codistributed primarily in the Golgi region. By yeast two-hybrid analysis, the binding site on G(alpha)i3 for calnuc was mapped to the C-terminal region because removal of the last 12 amino acids (but not 11) abolished the interaction. Peptide competition indicated that calnuc, with its coiled-coil domain constituted by the two EF-hands, binds to G(alpha)i3's C-terminal alpha5-helix. These results demonstrate that calnuc may play an important role in G protein- and Ca(2+)-regulated signal transduction events.