Sandbox Reserved 1701: Difference between revisions

The allosteric sodium site in class A GPCRs has been characterized as important in inactive state GPCR stabilization <ref name="Katritch">PMID:24767681</ref>. Katritch et al <ref name="Katritch">PMID:24767681</ref> describe that class A GPCRs lacking conserved D2.50 and other polar residues within the sodium pocket are typically inactive. The MRGPRX2 <scene name='90/904306/Sodium_site_2/1'>sodium binding site</scene> consists of conserved D2.50, or ASP-75, and GLY-116 compared to the [https://proteopedia.org/wiki/index.php/Neurotensin_receptor#sodium%20binding%20pocket previously conserved] polar residues in this binding pocket such as S3.39. Other class A GPCRs demonstrate a larger sodium binding pocket with a higher negative character allowing for a suitable environment for sodium ions to bind. In MRGPRX2, this sodium binding pocket lacks the same amount of <scene name='90/904306/Sodium_site_charge/2'>negative character</scene> with the shift to a glycine residue rather than serine. However, evidence suggests that sodium is still able to bind in X2's sodium binding site even with fewer conserved residues.

The E/DRY motif in most class A GPCRs is responsible for forming salt bridges with surrounding residues and TM6<ref name="Rovati">PMID: 17192495</ref>. These salt bridges maintain the inactive conformation of the receptor until ligand binding breaks the ionic "lock" from these interactions. MRGPRX2 has an <scene name='90/904306/Erc_motif_3/1'>ERC Motif</scene> rather than the typically [https://proteopedia.org/wiki/index.php/A_Physical_Model_of_the_%CE%B22-Adrenergic_Receptor#conserved%20DRY%20motif conserved E/DRY Motif]. The amino acid residue shift from TYR-174 to CYS-128 allows compaction of the helices in MRGPRX2 where the standard TYR physically pushes the TMD helices apart('''Figure 4'''). The conserved residues E and R still form salt bridges with nearby residues. This and the closer packing of the helices contribute to a less significant TMD conformational change upon ligand binding ('''Figure 10''').

The residues in the <scene name='90/904306/Npxxy_motif/4'>NPxxY Motif</scene> 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.

The <scene name='90/904306/Cpxw_motif_2/1'>CWxP Motif</scene> 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 by participating in the rearrangement of TM6 and TM7 upon receptor activation<ref name="Olivella>PMID:23497259</ref>. This residue's role is supported by its conservation in MRGPRX2 as it is a functional GPCR.