Illuminating insights into protein-protein interactions using bioluminescence resonance energy transfer (BRET)

Type 1 angiotensin receptor pharmacology: signaling beyond G proteins

Drugs that inhibit the production of angiotensin II (AngII) or its access to the type 1 angiotensin receptor (AT₁R) are prescribed to alleviate high blood pressure and its cardiovascular complications. Accordingly, much research has focused on the molecular pharmacology of AT₁R activation and signaling. An emerging theme is that the AT₁R generates G protein dependent as well as independent signals and that these transduction systems separately contribute to AT₁R biology in health and disease. Regulatory molecules termed arrestins are central to this process as is the capacity of AT₁R to crosstalk with other receptor systems, such as the widely studied transactivation of growth factor receptors. AT₁R function can also be modulated by polymorphisms in the AGTR gene, which may significantly alter receptor expression and function; a capacity of the receptor to dimerize/oligomerize with altered pharmacology; and by the cellular environment in which the receptor resides. Together, these aspects of the AT₁R “flavour” the response to angiotensin; they may also contribute to disease, determine the efficacy of current drugs and offer a unique opportunity to develop new therapeutics that antagonize only selective facets of AT₁R function.

A suite of tools and application notes for in vivo protein interaction assays using bioluminescence resonance energy transfer (BRET)

Bioluminescence resonance energy transfer (BRET) is a natural biophysical phenomenon that underlies an emerging technique to monitor protein-protein interactions in living cells in real time. Here, we present a series of technical advances to enhance the utility of the BRET assay in plants. A series of recombination cloning vectors was generated to accelerate the expression of proteins tagged with Renilla luciferase or yellow fluorescent protein under transient assay conditions and in stable transgenic plants. Working in stably transformed Arabidopsis or tobacco, we then detected BRET between three pairs of candidate interaction partners: dimerization of the E3 ubiquitin ligase COP1, interaction between COP1 and the B-box protein STH, and interaction between the light regulatory bZip transcription factors HY5 and HYH. A codon-optimized version of the Renilla luciferase gene resulted in improved expression in Arabidopsis. Renilla luciferase was active in a variety of subcellular organelles, including plastids, mitochondria, peroxisomes and Golgi stacks. In a survey of the Arabidopsis light signaling machinery as a model system, we estimated the likelihood that a known protein-protein interaction can be documented using BRET. Finally, we show that Renilla luciferase may serve as a reporter of protein stability in a cycloheximide chase assay.

Oligomeric structure of the alpha1b-adrenoceptor: comparisons with rhodopsin

The structural basis of the quaternary organization of rhodopsin has recently been explored and modeled. Because information obtained from studying rhodopsin has frequently been directly applicable to other G protein-coupled receptors we wished to ascertain if dimeric and/or oligomeric forms of the alpha(1b)-adrenoceptor could be observed and if so whether rhodopsin might provide insights into the quaternary structure of this receptor. Co-immunoprecipitation and both conventional and time-resolved fluorescence resonance energy transfer studies demonstrated quaternary structure of the alpha(1b)-adrenoceptor and, in concert with the reconstitution of fragments of this receptor, provided information on the molecular basis of these interactions. Development of three color fluorescence resonance energy transfer (FRET) allowed the imaging of alpha(1b)-adrenoceptor oligomers in single living cells. Mutation of hydrophobic residues in transmembrane domains I and IV of the receptor resulted in marked reduction in three color FRET suggesting an alteration in oligomeric organization and potential similarities with rhodopsin. The mutated alpha(1b)-adrenoceptor was unable to reach the cell surface, did not become terminally N-glycosylated and was unable to signal.

Materials for Fluorescence Resonance Energy Transfer Analysis: Beyond Traditional Donor–Acceptor Combinations

The use of Förster or fluorescence resonance energy transfer (FRET) as a spectroscopic technique has been in practice for over 50 years. A search of ISI Web of Science with just the acronym "FRET" returns more than 2300 citations from various areas such as structural elucidation of biological molecules and their interactions, in vitro assays, in vivo monitoring in cellular research, nucleic acid analysis, signal transduction, light harvesting and metallic nanomaterials. The advent of new classes of fluorophores including nanocrystals, nanoparticles, polymers, and genetically encoded proteins, in conjunction with ever more sophisticated equipment, has been vital in this development. This review gives a critical overview of the major classes of fluorophore materials that may act as donor, acceptor, or both in a FRET configuration. We focus in particular on the benefits and limitations of these materials and their combinations, as well as the available methods of bioconjugation.

The visible touch: in planta visualization of protein-protein interactions by fluorophore-based methods

Non-invasive fluorophore-based protein interaction assays like fluorescence resonance energy transfer (FRET) and bimolecular fluorescence complementation (BiFC, also referred to as "split YFP") have been proven invaluable tools to study protein-protein interactions in living cells. Both methods are now frequently used in the plant sciences and are likely to develop into standard techniques for the identification, verification and in-depth analysis of polypeptide interactions. In this review, we address the individual strengths and weaknesses of both approaches and provide an outlook about new directions and possible future developments for both techniques.

Opioid and cannabinoid receptors: friends with benefits or just close friends?

mu-Opioid (MOP) and cannabinoid CB1 receptors mediate overlapping pharmacological responses in clinically important areas such as drug abuse and pain management, and functional interactions between agonists at these receptors have long been recognized. In the present issue of this Journal, Rios and co-workers have provided the first strong evidence that the two receptors interact directly when coexpressed in the same cells. The authors report a close physical association between MOP and CB1 receptors and novel pharmacological interactions of MOP and CB1 agonists. They argue that MOP/CB1 heterodimer formation explains these interactions. If correct, the direct interaction of MOP and CB1 pharmacophores in a quaternary complex would provide real benefits by opening the potential for development of novel MOP/CB1 small molecules or new strategies for use of current ligands. However, a lot more evidence will be required before the heterodimer interpretation can be accepted. If it turns out that MOP and CB1 receptors do not readily form hetero-oligomers, the study by Rios and co-workers shows that they are still friends but there may be few benefits.