Variant amino acids in different domains of the human melanocortin 1 receptor impair cell surface expression

The alpha melanocyte-stimulating hormone receptor (MC1R) is a heptahelical G protein-coupled receptor (GPCR) found in the plasma membrane of melanocytes. By mediating the melanogenic response to melanocortins, MC1R is a major determinant of mammalian pigmentation. The human MC1R gene is unusually polymorphic. Many loss-of-function alleles have been described, but the molecular basis for their functional impairment remains most often unknown. Here we report a study of two natural MC1R loss-of-function variants, Leu93Arg and Arg162Pro, and two artificial mutants, Cys35Ala and a deleted form missing the last five amino acids in the carboxyl tail. When expressed in HEK 293T cells, those mutants neither bound an iodinated hormone analogue nor elicited cAMP increases in response to saturating doses of a superpotent agonist. Cell surface expression of mutant receptors was dramatically decreased respect to the wild type form, in spite of smaller changes in total protein abundances and intracellular stability. Accordingly, aberrant processing with intracellular retention is the most likely cause of loss-of-function for those mutants. Therefore, mutations in virtually any region of the heptahelical protein, including its extracellular N terminus, a transmembrane fragment, intracellular loops or carboxyl terminal cytosolic extension, seem to compromise normal MC1R processing.