RESUMO
Large membrane proteins such as G protein-coupled receptors (GPCRs) are difficult for NMR study due to severe signal overlaps and unfavorable relaxation properties. We used a trimethylsilyl (TMS) group as a reporter group for 1 H NMR study of conformational changes in proteins, utilizing high-intensity 1 H NMR signals near 0 p.p.m. The ß2 -adrenergic receptor was labeled with TMS groups at two cysteines located at the cytoplasmic ends of helices VI and VII. Binding of various ligands led to changes in 1 H NMR signals, which manifested that helix VI is sensitive to G protein-specific activation, whereas helix VII is sensitive to ß-arrestin-specific activation. Thus, the TMS group is a useful reporter group in NMR for studying conformational changes in membrane proteins such as GPCRs.
Assuntos
Espectroscopia de Ressonância Magnética/métodos , Modelos Moleculares , Receptores Adrenérgicos beta 2/metabolismo , Humanos , Ligantes , Conformação Proteica , Receptores Adrenérgicos beta 2/química , Receptores Acoplados a Proteínas G/química , Receptores Acoplados a Proteínas G/metabolismo , Compostos de Trimetilsilil/químicaRESUMO
The amino acid 4-fluoro-L-phenylalanine (4F-Phe) was introduced at the positions of Phe6 and Phe22 in the 29-residue polypeptide hormone glucagon by expressing glucagon in E. coli in the presence of an excess of 4F-Phe. Glucagon regulates blood glucose homeostasis by interaction with the glucagon receptor (GCGR), a class B GPCR. By referencing to the 4F-Phe chemical shifts at varying D2O concentrations, the solvent exposure of the two Phe sites along the glucagon sequence was determined, showing that 4F-Phe6 was fully solvent exposed and 4F-Phe22 was only partially exposed. The incorporation of fluorine atoms in polypeptide hormones paves the way for novel studies of their interactions with membrane-spanning receptors, specifically by differentiating between effects on the solvent accessibility, the line shapes, and the chemical shifts from interactions with lipids, detergents and proteins. Studies of interactions of GCGR with ligands in solution is at this point of keen interest, given that recent crystallographic studies revealed that an apparent small molecule antagonist actually binds as an allosteric effector at a distance of ~20 Å from the orthosteric ligand binding site (Jazayeri et al., in Nature 533:274-277, 2016).