Amino acid residues in third intracellular loop of melanocortin 1 receptor are involved in G-protein coupling.

To delineate domains essential for G-protein coupling in melanocortin 1 receptor (MC1R), we mutated polar and basic residues to alanine at eleven positions in the putative third intracellular loop and determined consequent changes in the ligand binding and generation of second messenger cAMP. Results demonstrate that ligand binding affinity was not affected by any of the mutations. However, every mutant displayed reduced functional response as compared to the wild type receptor. Replacement of residues (K226, R227, Q228, R229, H232, Q233 and K238) present in second half of third intracellular loop resulted in an almost complete loss of functional response. The results have demonstrated that the amino acid residues present in C-terminal portion of third intracellular loop of MC1R are involved in coupling to G-protein and that a region of four amino acids, K226-R227-Q228-R229 is essential for coupling of MC1R to G-protein.