MRGPRX2 is a certain type of GPCR that is located in the cellular membranes of mast cells.
Figure 1. MRGPRX2 as it sits within the cellular membrane. Phospholipid bilayer is represented by grey dots, with labeled cellular locations
Background
GCPR’s or G-Protein Coupled Receptors are a special type of protein receptor that promotes cellular signaling. Due to its structure spanning the cellular membrane, it works to transmit extracellular signals to create a change inside the cell. This is called signal transduction, and is a common way extracellular signals produce a change within cells. Some common cellular pathways that utilize GPCR’s are found in Rhodopsin, a protein essential for the human vision response, or the adrenaline fight-or-flight response. Understanding GPCR’s and how they produce their desired intracellular signal is essential understanding essential cellular pathways, especially in their diseased states. As of 2017, there were about 475 drugs in circulation that acted on 108 GPCR targets, and at least 321 GPCR-targeting drugs were in clinical trial stages, with about 20% of them targeting novel GPCR’s. [1] Because of the clinical relevance of GPCR’s, every new structure found, such as MRGPRX2, contributes to essential drug development to both treat disease, or modulate harmful side effects.
Because of how many different types of GPCR’s there are, they have been categorized into 6 different classes based on shared sequences and functions.[2] MRGPRX2 is categorized into the Class A receptor family, however it has important differences that make it a unique type of Class A receptor. [3][4]
GPCR Structure
Cryo-electron microscopy (cryo-EM) was used to image the MRGPRX2 receptor to analyze its structure. [3] [4] which helped to classify it into the A family of GPCR's. MRGPRX2 therefore shares the same general structural domains of all GPCR's. This includes a and a domain. The G-protein domain consists of , , and subunits. In preparing the protein sample, MRGPRX2 was prepared with the in order to stabilize the membrane for proper imaging. For simplicity and to focus on the specific MRGPRX2 receptor, the antibody has been removed.
Transmembrane Domain
The transmembrane domain spans the cellular membrane. It consists of and (three extracellular loops, and three intracellular loops). The transmembrane helices are numbered 1-7 and contain special conserved motifs that are shared across other A family receptors. These motifs are expanded upon later, as they heavily contribute to the structure and therefore function of the transmembrane domain as a whole.
The extracellular domain of the protein is responsible for ligand binding, which initiates signal transduction. The properties of the extracellular environment of the transmembrane domain determines what ligands bind to the protein, and what type of binding interactions the protein and ligand have. In MRGPRX2, the extracellular side of the protein has one binding pocket with . Sub-pocket 1 is negatively charged due to negatively charged residues (D184 and E164), while sub-pocket 2 contains hydrophobic amino acids which contribute to hydrophobic interactions between the ligand and protein.
The intracellular domain is what connects the transmembrane and G-protein domains together. There are a wide variety of residues and important interactions that contribute to this interaction, and it is very important in transmitting the extracellular signal of ligand binding to the intracellular environment where the G-protein binds and can become activated.
G-Protein
GTP-binding proteins, also known as G-proteins, are heterotrimeric complexes consisting of alpha, beta, and gamma subunits that interact with membrane receptor proteins. G-proteins are responsible transmitting extracellular signals into the cell upon activation. This activation happens when the alpha subunit of the G-protein binds GTP instead of GDP, and then disassociates from the rest of the protein, initiating the intracellular signaling cascade. There are different families of G-alpha subunits, Gαi, Gαs, Gα12/13, and Gαq [5]. MRGPRX2 has been found to bind both Gαi and Gαq subunits with relatively no major structural changes between the two despite slightly different amino acids present [3] [4]. Throughout this page, MGPRX2 is always shown with Gq. Figure 2a shows the overlay of MGPRX2 with either the Gq or Gi alpha subunit. Figure 2b shows the specific residues involved in the between the membrane receptor and G protein. The major difference between the two comes from one amino acid difference (valine on Gq versus phenylalanine on Gi) that pushes the Gi subunit 2Å away from the arginine residue on helix 6 of the transmembrane protein. This is the only major structural difference between Gq and Gi subunits.
Figure 2a. Overlay of MGPRX2-Gq (red-dark blue) and MGPRX2-Gi (cyan-yellow).
Figure 2b. Important residues involved in the interface between MGPRX2 and Gq/ Gi subunits. Arrow pointing to the major difference between the interfaces, which comes from the final C-terminus residue on the G-alpha subunit. In Gq, there is a valine while in Gi, there is a phenylalanine. This pushes the Gi subunit 2Å away from the arginine residue on helix 6 of the transmembrane protein.
Novel Characteristics
The most important findings about the MRGPRX2 receptor are the differences between it and all other previously discovered GPCR's found in the A family. Many conserved structural motifs, characteristic of the A family receptors, are absent on MRGPRX2. These structural motif differences contribute to a membrane surface ligand binding rather than a ligand binding deep within the helices (Figure 3). To demonstrate this difference in depth binding, MRGPRX2 is compared to 5-HT2AR, another class A GPCR with more conserved structural motifs.
Figure 3. Comparison of ligand Cortistatin-14 binding in MRGPRX2 (left) and binding in 5HT2AR (right)
Toggle Switch
Certain residues that lie in the ligand binding pocket can act as molecular switches to turn the GPCR “on” or “off” and are fittingly called toggle switches. Toggle switches in receptors are essential in interacting with the ligand upon binding, as they can then initiate the transmission of the molecular signal through the protein. Trp-336 has been known to act as this "iconic" toggle switch in GPCR’s of the A family [6], and is also an important residue in another motif, known as the CWxP motif. However in MRGPRX2, the residue has been replaced to a [3] [4]. This is a significant modification to the receptor structure. By replacing the large tryptophan residue with a small glycine, the membrane helicies, especially helix 7 on which the toggle switch is found, can pack closer together. The ligands that interact with MRGPRX2 then bind much of the receptor, as opposed to deeper within the helices. This significant change can be visualized in Figure 3. This significantly changes what structures of ligands are able to interact with this receptor and therefore what types of molecules can activate the Human Itch GPCR. More details about what kinds of ligands bind to this receptor are discussed later.
Sodium Site
The MRGPRX2 consists of ASP-75 and GLY-116 compared to the previously conserved residues in this binding pocket. Other class A GPCRs demonstrate a larger binding pocket with a higher negative character allowing for a suitable environment for sodium ions to bind. In MRGPRX2, this pocket lacks the same amount of with the shift to a glycine residue rather than typical negative residues. The helices for the MRGPRX2 in the binding pocket are also more collapsed making this pocket less accessible for sodium ions.
PIF/LLF Motif
Another motif found in most, but not all, A family GPCR’s is the PIF motif. The three residues are found on transmembrane helices 5, 3, and 6, respectively. In MRGPRX2, the PIF motif is changed to LLF residues. Figure 3 shows the conserved PIF motif on 5HT2AR, compared to the LLF motif on MRGPRX2, found at on transmembrane helices 5, 3, and 6, respectively. This contributes to shifting helix 6 towards helix 3, and contributes to the tighter packing of helices [3] [4] and therefore a more surface-level ligand binding site.
Figure 3. Conserved PIF motif in 5HT2AR (teal) compared to the LLF motif found in MRGPRX2 (red). Transmembrane helices and residues are numbered and labeled to show how this structural change shifts the orientation of the helices.
DRY/ ERC Motif
MRGPRX2 has an rather than the typically conserved E/DRY Motif. The amino acid residue shift from TYR-174 to CYS-128 has spatial arrangement implications where the helices are more compact in MRGPRX2 without the TYR to physically push the TMP helices apart.
Disulfide Bonds
The MRGPRX2 disulfide bond is between on TM helices 5 and 4, respectively. In other class A GPCRs, this disulfide bond is between and extracellular loop (ECL2) and a TM helix (TM3). For example, the shows this disulfide bond between the ECL2 and TM3. This different disulfide bond location contributes to surface level binding of ligands.
Figure 5. Overlay of the 5HT2AR and MRGPRX2 TMP for comparison of disulfide bond location.
Further Information
The MRGPRX2 GPCR also contains semi-conserved or fully-conserved motifs that are seen in other class A GPCRs.
NPxxY Motif
The residues in the are pivotal for receptor activation in all Class A GPCRs. This motif is conserved in the MRGPRX2 receptor with residues VAL-231, ASP-75, ASN-275, and TYR-279.
CWxP Motif
The is almost fully conserved except for TRP-236, or the toggle switch, which is replaced with GLY-236. CYS-235, LEU-237, and PRO-238 are all conserved. CYS6.47 may play a fundamental role in GPCR activity which would be supported by its conservation in MRGPRX2[7].
Ligands
MRGPRX2 binds a wide range of small molecule and peptide ligands. These ligands have positively charged regions which can interact with the negative binding pocket, sub-pocket 1. Some of these larger ligands also have a hydrophobic region which can interact with sub-pocket 2. MRGPRX2 has been known to interact with PAMP-12, Cortistatin-14, C48/80, and Zinc-3573[4],[3].
Common MRGPRX2 ligands with positive regions in blue, negative regions in red, and hydrophobic regions in green
Ligands' charge or hydrophilicity are shown to demonstrate where they may interact with sub-pockets 1 and 2. , specifically, interacts with sub-pocket 1 through LYS-3 and sub-pocket 2 through LYS-8.
Function
GPCRs undergo a transmembrane protein conformational change upon ligand binding. This signal is then transduced to the G-protein allowing for downstream responses due to between the alpha subunit of the G-protein and the transmembrane protein.
Before Activation
The culmination of different motifs observed in MRGPRX2 compared to other class A GPCRs leads to external membrane ligand binding. The MRGPRX2 GPCR undergoes a much smaller conformational change upon ligand binding compared to other Class A GPCRs due to surface level binding versus deep helix binding. This contrast is represented in the photo below to show the unbound and bound transmembrane proteins of MRGPRX2 and 5-HT2A.
Figure 10. Overlay of unbound (transparent) and bound (opaque) transmembrane proteins of both MRGRPX2 (left) and 5-HT2AR (right).
After Activation
After ligand binding and transmembrane protein activation, this signal is sent to the alpha subunit of the G-protein which undergoes its own chemical and conformational changes. The alpha subunit is initially bound with GDP which then is physically replaced by GTP leading to conformational changes. These changes can be seen in this video of a G-protein alpha subunit activation derived from common rats.
Clinical Relevance
The commonly observed MRGPRX2 ligands also share similar structural features with targeted drugs for clinical relevance. MRGPRX2 has been identified as a potential target for drugs such as Atracurium, Rocuronium, Ciprofloxacin, and Levofloxacin[8]. These drugs have similarly charged regions which may interact with sub-pockets 1 and/or 2. By binding these target drugs, MRGPRX2 can elicit or prevent a response to aid with itch-related side effects.
3D Structures
7s8l, MRGPRX2 Gq
7s8m, MRGPRX2 Gi
6wha, 5HT2AR
