Melanocortin-2 receptor accessory protein MRAP forms antiparallel homodimers
Open accessAbstract
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 other melanocortin, beta2-adrenergic, or TSH-releasing hormone receptors. MRAP is the first eukaryotic membrane protein identified with an antiparallel homodimeric structure.
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MRAP is necessary for trafficking and cell surface expression of MC2R, as well as ACTH binding and signal transduction.
“Melanocortin receptor accessory protein is a highly unusual protein in that it naturally exists as an antiparallel homodimer and seems to be necessary for trafficking and cell surface expression of the MC2R, as well as binding of ACTH and hence signal transduction (36, 37).”
From ACTH Antagonists (2016) · cites this paper as [36]
The Results (page not provided) show that MRAP permits MC2R trafficking to the cell surface and is required for ACTH-stimulated cAMP signaling. The paper does not establish separately that MRAP is necessary for ACTH binding to MC2R. (LLM, conf=0.85)
Experimental — accuracy not yet measuredMC2R is absolutely dependent on MRAP for cell surface expression and function.
“The ACTH receptor, also known as the melanocortin 2 receptor (MC2R), is unusual in that it is absolutely dependent on a small accessory protein, melanocortin receptor accessory protein (MRAP) for cell surface expression and function.”
From ACTH Antagonists (2016) · cites this paper as [36]
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Minireview: the melanocortin 2 receptor accessory proteins
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“Melanocortin receptor accessory protein is a highly unusual protein in that it naturally exists as an antiparallel homodimer and seems to be necessary for trafficking and cell surface expression of the MC2R, as well as binding of ACTH and hence signal transduction (36, 37).”
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- Authors
- Sebag, J. A., & Hinkle, P. M.
- Affiliations
- University of Rochester Medicine (US)
- ORCID
- Sebag, Julien A.
- Venue
- Proceedings of the National Academy of Sciences
- Publisher
- National Academy of Sciences
- Published
- 2007-12-18
- Volume
- 104
- Issue
- 51
- Pages
- 20244-20249
- ISSN
- 0027-8424 (ISSN-L)
- 1091-6490
- PMC
- PMC2154416
- OpenAlex
- W2123276616
- Cited by
- 168
- References
- 24
- Citations by year
- 2026: 5 · 2025: 3 · 2024: 1 · 2023: 10 · 2022: 14 · 2021: 8 · 2020: 9 · 2019: 11 · 2018: 7 · 2017: 15 · 2016: 10 · 2015: 6 · 2014: 11 · 2013: 12 · 2012: 10
- FWCI
- 3.57
- Type
- journal article
- Language
- en
- Open access
- green
- www.ncbi.nlm.nih.gov
- Locations
- Proceedings of the National Academy of Sciences (publishedVersion)
- PubMed Central (publishedVersion)
- PubMed (publishedVersion)
- Funders
- National Institute of Diabetes and Digestive and Kidney Diseases (DK-19974, R01DK019974)
- Topics
- Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Receptor Mechanisms and Signaling
- Life Sciences › Neuroscience › Cellular and Molecular Neuroscience › Neuropeptides and Animal Physiology
- Life Sciences › Biochemistry, Genetics and Molecular Biology › Cell Biology › Cellular transport and secretion
- Keywords
- melanocortin 2 receptor, MRAP, membrane topology, N-terminal glycosylation, receptor trafficking, G protein-coupled receptors, ACTH signaling, endoplasmic reticulum retention, adrenal cells
- MeSH terms
- Amino Acid Sequence, Animals, Cell Membrane, Cricetulus, Cricetinae, Humans, Membrane Proteins, Molecular Sequence Data, CHO Cells, Protein Structure, Tertiary, Dimerization, Receptor, Melanocortin, Type 2
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