title: Mutationally induced disulfide bond formation within the third extracellular loop causes melanocortin 4 receptor inactivation in patients with obesity.
abstract: By screening patients with severe early onset obesity for mutations within the melanocortin 4 receptor (MC4R) gene, we have identified a missense mutation (C271R) that occurs homozygous in two siblings with obesity. In-depth functional characterization of C271R revealed a right-shifted concentration response curve due to lower affinity to natural and synthetic MC4R agonists and a reduced cell surface expression. Cys-271 is located in the third extracellular loop. Here, we provide evidence that Cys-271 forms an intra-loop disulfide bond with Cys-277. Unexpectedly, we found that loss of receptor function is not only caused by the disruption of this disulfide bridge. Our data strongly support a new mechanism in which the receptor malfunction in the C271R mutant is induced by formation of a functionally disastrous disulfide bridge between Cys-277 and a third Cys residue at position 279. Mutational and chemical disruption of this improper disulfide bond was able to restore normal receptor potency. By demonstrating that a loss of a disulfide bond-participating Cys residue can favor a functionally disastrous disulfide bond, we now add a new mechanism of how Cys residues can be involved in G-protein-coupled receptor malfunction.
mesh_terms: D000293:Adolescent; D000328:Adult; D000595:Amino Acid Sequence; D002648:Child; D004220:Disulfides / Q000737:chemistry*; D005260:Female; D006801:Humans; D008297:Male; D008969:Molecular Sequence Data; D020125:Mutation, Missense; D009765:Obesity / Q000378:metabolism*; D010375:Pedigree; D011869:Radioligand Assay; D044105:Receptor, Melanocortin, Type 4 / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
chemical_list: D004220:Disulfides; D044105:Receptor, Melanocortin, Type 4