Cell Res: Molecular mechanism by which ligands and helper proteins work together to regulate melanocortisol receptor 2

The function of G protein-coupled receptors (GPCRs) depends on the regulation of a variety of helper proteins, such as single-shot transmembrane protein receptor activity modification protein 1 (RAMP1) allosteric regulation of class B GPCR activity
.
The melanocortin receptor accessory protein family (MRAP) consists of two members (MRAP1 and MRAP2), of which MRAP1 is considered an essential element
for melanocortisol receptor 2 (MC2R) transport to the plasma membrane and generate activity.
Compared with the regulation of class B GPCRs by RAMP1, the scientific community has little
understanding of the mechanism of auxiliary proteins such as MRAPs in regulating class A GPCRs.

MC2R is the most unique member of the five receptor subtypes of the melanocortin receptor (MCR) family, only its activity depends on the helper protein MRAP1, and it has selective specificity
for the ligand adrenocorticotropic hormone (ACTH).
Early research suggests that the N-terminus of MRAP1 is necessary to activate MC2R
.
In addition, the "information" motif HFRW and the "address" motif KKRR contained in the ligand ACTH are both considered necessary for ACTH to bind to MC2R, but the mechanism of how MRAP1 and ACTH co-activate MC2R remains unclear
.

MC2R is a key receptor in the hypothalamic-pituitary-adrenal (HPA) axis that regulates stress responses, stimulates glucocorticoid production and cortisol secretion in physiological states, and dysfunction can lead to glucocorticoid deficiencies such as familial glucocorticoid deficiency (FGD), or hypercortisol such as congenital adrenal hyperplasia (CAH) and Cushing's syndrome (CS).

。 Therefore, revealing the molecular mechanism of selective recognition of MC2R by ACTH will also provide a structural basis and new ideas
for understanding the physiological process of peptide hormones regulating MC2R and the design of drugs targeting MC2R.

On January 2, 2023, Xu Huaqiang's team from the Shanghai Institute of Materia Medica, Chinese Academy of Sciences, together with Jiang Yi from Lingang Laboratory and Wang Mingwei's team from Fudan University, published a research paper
entitled "Structural basis of signaling regulation of the human melanocortin-2 receptor by MRAP1" in Cell Research 。 The researchers used cryo-EM to analyze the three-dimensional structure of MC2R with MRAP1, ACTH and downstream Gs protein complexes for the first time, revealing the molecular recognition mechanism
of MC2R-dependent MRAP1 for ACTH.

MRAP1 binds between TM5 and TM6 of MC2R in the form of a single transmembrane with the N-terminal facing outward, and ACTH is bound to
the binding pocket of MC2R in the shape of a fishhook 。 Through the comparative analysis of ligand-binding pockets with existing structures MC1R and MC4R, it was found that the pockets of MC2R-bound ACTH were different in charge and hydrophobicity, and the binding mode of the "information" motif HFRW in ACTH to receptors was also different, resulting in the instability of ACTH in the binding pocket of MC2R, while the N-terminal loop of MRAP1 stabilized ACTH in the binding pocket in a belt-like binding mode to further activate MC2R
。 This study elucidates the molecular mechanism by which MC2R relies on MRAP1 activation and specifically recognizes ACTH, revealing a novel "seat belt model"
for helper proteins to regulate G protein-coupled receptor function.
These findings will deepen the understanding of the mechanism of action of GPCR helper proteins and provide implications
for the development of selective MC2R drugs.

The cryo-EM data in this study were collected
by the cryo-EM platform of the Shanghai Institute of Materia Medica, Chinese Academy of Sciences.
Luo Ping, a postdoctoral fellow at Shanghai Institute of Materia Medica, and Feng Wenbo, a doctoral student at Fudan University, are the co-first authors of the paper, Jiang Yi, Xu Huaqiang and Wang Mingwei are co-corresponding authors, and Shanghai Institute of Materia Medica
.
The work has been funded
by the National Natural Science Foundation of China, the Ministry of Science and Technology, and the Shanghai Municipal Science and Technology Major Project.

Figure 1.
Ligand selectivity of MC2R and cryo-EM structure
of complex.
a-b.
Sequence comparison of melanocorticin receptor ligands and selectivity
for ligands.
c-d.
Cryo-EM density diagram and cartoon diagram
of ACTH-MC2R-Gs-MRAP1 complex.
e.
Structural comparison
of existing melanocorticoreceptor structures.

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5pt;" _msthash="101746" _msttexthash="1953861">(Contributing department: Xu Huaqiang research group)