Abstract
The melanocortin 1 receptor (MC1R), a Gs protein-coupled receptor (GPCR) expressed in melanocytes, is a major determinant of skin pigmentation and phototype. MC1R activation stimulates melanogenesis and increases the ratio of black, strongly photoprotective eumelanins to reddish, poorly photoprotective pheomelanins. Several MC1R alleles are associated with red hair, fair skin, increased sensitivity to ultraviolet radiation (the RHC phenotype) and increased skin cancer risk. Three highly penetrant RHC variants, R151C, R160W, and D294H are loss-of-function MC1R mutants with altered cell surface expression. In this study, we show that forward trafficking was normal for D294H. Conversely, export traffic was impaired for R151C, which accumulated in the endoplasmic reticulum (ER), and for R160W, which was enriched in the cis-Golgi. This is the first report of steady-state retention in a post-ER secretory compartment of a GPCR mutant found in the human population. Residues R151 and R160 are located in the MC1R second intracellular loop (il2). Two other mutations in il2, T157A preventing T157 phosphorylation and R162P disrupting a (160)RARR(163) motif, also caused intracellular retention. Moreover, T157 was phosphorylated in wild-type MC1R and a T157D mutation mimicking constitutive phosphorylation allowed normal traffic, and rescued the retention phenotype of R160W and R162P. Therefore, MC1R export is likely regulated by T157 phosphorylation and the (160)RARR(163) arginine-based motif functions as an ER retrieval signal. These elements are conserved in mammalian MC1Rs and in all five types of human melanocortin receptors. Thus, members of this GPCR subfamily might share common mechanisms for regulation of plasma membrane expression.