Melanocortin-2 receptor accessory protein MRAP forms antiparallel homodimers

The melanocortin-2 (MC2) receptor accessory protein (MRAP) is required for trafficking of the G protein-coupled MC2 receptor to the plasma membrane. The mechanism of action and structure of MRAP, which has a single transmembrane domain, are unknown. Here, we show that MRAP displays a previously uncharacterized topology. Epitopes on both the N- and C-terminal ends of MRAP were localized on the external face of CHO cells at comparable levels. Using antibodies raised against N- and C-terminal MRAP peptides, we demonstrated that both ends of endogenous MRAP face the outside in adrenal cells. Nearly half of MRAP was glycosylated at the single endogenous N-terminal glycosylation site, and over half was glycosylated when the natural glycosylation site was replaced by one in the C-terminal domain. A mutant MRAP with potential glycosylation sites on both sides of the membrane was singly but not doubly glycosylated, suggesting that MRAP is not monotopic. Coimmunoprecipitation of differentially tagged MRAPs established that MRAP is a dimer. By selectively immunoprecipitating cell surface MRAP in one or the other orientation, we showed that MRAP homodimers are antiparallel and form a stable complex with MC2 receptor. In the absence of MRAP, MC2 receptor was trapped in the endoplasmic reticulum, but with MRAP, the MC2 receptor was glycosylated and localized on the plasma membrane, where it signaled in response to ACTH. MRAP acted specifically, because it did not increase surface expression of melanocortin, beta 2-adrenergic, or TSH-releasing hormone receptors. MRAP is the first eukaryotic membrane protein identified with an antiparallel homodimeric structure.