Basic Amino Acids at the C-Terminus of the Third Intracellular Loop Are Required for the Activation of Phospholipase C by Cholecystokinin-B Receptors

Abstract: In common with other G_q protein-coupled receptors, the third intracellular loop of the cholecystokinin-B (CCK-B) receptor contains three basic amino acids (K333/K334/R335) at the C-terminal segment. To determine the importance of these conserved basic residues in G_q-protein activation and stimulation of phospholipase C, these basic amino acids were mutated. Subsequently, the ability of resulting mutant receptors to activate phospholipase C was investigated by measuring inositol phosphate formation in COS-7 cells and recording Ca²⁺-activated Cl^? currents from Xenopus oocytes. Site-directed mutagenesis was performed to mutate the three basic amino acids, K333/K334/R335, to neutral amino acids, M333/T334/L335. When the resulting mutant CCK-B receptors were expressed in COS-7 cells and Xenopus oocytes, sulfated cholecystokinin octapeptide (CCK-8) failed to induce inositol phosphate formation in COS-7 cells and evoke Ca²⁺-activated Cl^? currents from oocytes. Each basic amino acid was also mutated (K333M, K334T, and R335L). All three single-point mutations resulted in a significant reduction in CCK-8-induced inositol phosphate formation and CCK-8-activated Ca²⁺-dependent Cl^? currents. It is interesting that substituting the basic amino acids, K333/K334/R335, with three other basic residues, R333/R334/K335, did not change the maximal CCK-8-simulated inositol phosphate formation and the amplitude of CCK-8-evoked Ca²⁺-dependent Cl^? currents. Radioligand-binding studies showed that the above-mentioned mutations did not affect the affinity for CCK-8 and receptor expression level in COS-7 cells. These findings suggest that basic amino acids at the C-terminus of the third cytoplasmic loop are required for the signal transduction by CCK-B receptors.