Sweet taste receptor interacting protein CIB1 is a general inhibitor of InsP3-dependent Ca2+ release in vivo

In a search for sweet taste receptor interacting proteins, we have identified the calcium- and integrin-binding protein 1 (CIB1) as specific binding partner of the intracellular carboxyterminal domain of the rat sweet taste receptor subunit Tas1r2. In heterologous human embryonic kidney 293 (HEK293) cells, the G protein chimeras G alpha(16gust44) and G alpha(15i3) link the sweet taste receptor dimer TAS1R2/TAS1R3 to an inositol 1,4,5-trisphosphate (InsP(3))-dependent Ca(2+) release pathway. To demonstrate the influence of CIB1 on the cytosolic Ca(2+) concentration, we used sweet and umami compounds as well as other InsP(3)-generating ligands in FURA-2-based Ca(2+) assays in wild-type HEK293 cells and HEK293 cells expressing functional human sweet and umami taste receptor dimers. Stable and transient depletion of CIB1 by short-hairpin RNA increased the Ca(2+) response of HEK293 cells to the InsP(3)-generating ligands ATP, UTP and carbachol. Over-expression of CIB1 had the opposite effect as shown for the sweet ligand saccharin, the umami receptor ligand monosodium glutamate and UTP. The CIB1 effect was dependent on the thapsigargin-sensitive Ca(2+) store of the endoplasmic reticulum (ER) and independent of extracellular Ca(2+). The function of CIB1 on InsP(3)-evoked Ca(2+) release from the ER is most likely mediated by its interaction with the InsP(3) receptor. Thus, CIB1 seems to be an inhibitor of InsP(3)-dependent Ca(2+) release in vivo.