Single-molecule spectroscopy of the β ₂ adrenergic receptor: Observation of conformational substates in a membrane protein

Abstract

Single-molecule studies of the conformations of the intact ␤2 adrenergic receptor were performed in solution. Photon bursts from the fluorescently tagged adrenergic receptor in a micelle were recorded. A photon-burst algorithm and a Poisson time filter were implemented to characterize single molecules diffusing across the probe volume of a confocal microscope. The effects of molecular diffusion and photon number fluctuations were deconvoluted by assuming that Poisson distributions characterize the molecular occupation and photon numbers. Photon-burst size histograms were constructed, from which the source intensity distributions were extracted. Different conformations of the ␤2 adrenergic receptor cause quenching of the bound fluorophore to different extents and hence produce different photon-burst sizes. An analysis of the photon-burst histograms shows that there are at least two distinct substates for the native adrenergic membrane receptor. This behavior is in contrast to one peak observed for the dye molecule, rhodamine 6G. We test the reliability and robustness of the substate number determination by investigating the application of different binning criteria. Conformational changes associated with agonist binding result in a marked change in the distribution of photon-burst sizes. These studies provide insight into the conformational heterogeneity of G protein-coupled receptors in the presence and absence of a bound agonist.G protein-coupled receptors (GPCRs) represent one of the largest families of integral membrane proteins, and GPCRs are responsible for most transmembrane signal transduction by hormones and neurotransmitters, as well as for the senses of vision, smell, and taste. GPCRs are characterized by seven transmembrane (TM) domains with an extracellular N terminus and a cytoplasmic C terminus (1). For many GPCRs, such as the ␤ 2 adrenergic receptor (␤ 2 AR), small molecular weight ligands bind to sites within the hydrophobic core formed by the TM ␣-helices. The ␤ 2 AR ( Fig. 1) belongs to the major subfamily of GPCRs that includes rhodopsin, for which a high-resolution crystal structure is now available (2). The binding of agonist ligands to a receptor induces conformational changes that facilitate interactions between the receptor and its cognate G protein.We have developed a method for site-specifically labeling Cys-265 in the ␤ 2 AR with fluorescein-5-maleimide (FM; ref. 3) to yield what we call FM␤ 2 AR. Cys-265 is adjacent to a G protein-coupling domain at the end of TM6 (see Fig. 1). In ensemble measurements, we demonstrated previously that FM bound to Cys-265 sensitively reports conformational changes, with agonists inducing a decrease in the fluorescence intensity of FM␤ 2 AR in proportion to the biological efficacy of the agonist. By using exogenous quenchers localized to different environments, we determined that during agonist-induced activation, the domain adjacent to Cys-265 rotates and͞or tilts to a more buried environment, which is closer to both the surface of the micellar compa...

Agonist-specific Transactivation of Phosphoinositide 3-Kinase Signaling Pathway Mediated by the Dopamine D2 Receptor

“…In rhodopsin, for example, the existence of multiple conformers is evident from absorbance changes (9). Multiple receptor conformational states are also evident in single molecule spectroscopy studies of the ␤2-adrenergic receptor (11) and are supported by the presence of phenotypically different serotonin 5HT2C receptor activation mutants (12). Pharmacological evidence for signal trafficking has been reported in several heptahelical receptors (14,15,66,67).…”

Conserved Helix 7 Tyrosine Acts as a Multistate Conformational Switch in the 5HT2C Receptor

“…Activation occurs by transition through intermediate conformations (38), with the equilibrium between these forms showing a characteristic pH sensitivity (39). The existence of multiple receptor conformers is also evident in single molecule spectroscopy studies of the ␤ adrenergic receptor (40). Pharmacological studies suggest that the existence of distinct receptor conformers can have functional significance.…”

Ligands and signaling proteins govern the conformational landscape explored by a G protein-coupled receptor

“…Indeed, many reports point at a model where GPCRs are highly dynamic proteins capable of adopting a large number of conformational states that signal with different efficacies to various pathways (3). The most detailed description of the conformational repertoire that a GPCR can adopt has been obtained in the case of the β 2 -adrenergic receptor with an experimental demonstration of a complex conformational landscape where ligands with distinct pharmacological properties shift the conformation of the receptor from one state to another (4)(5)(6)(7)(8)(9)(10)(11). In the same way, we showed that ligands with different efficacies stabilize different conformations of the serotonin receptor (12).…”