Structural analysis by NMR of G protein-coupled receptors (GPCRs) has proven to be extremely challenging. To reduce the number of peaks in the NMR spectra by segmentally-labeling a GPCR, we have developed a Guided Reconstitution method that includes the use of charged residues and Cys activation to drive heterodimeric disulfide bond formation. Three different cysteine-activating reagents: 5-5’-dithiobis(2-nitrobenzoic acid) [DTNB], 2,2’-dithiobis(5-nitropyridine) [DTNP] and 4,4’-dipyridyl disulfide [4-PDS] were analyzed to determine their efficiency in heterodimer formation at different pHs. Short peptides representing the N-terminal (NT) and C-terminal (CT) regions of the first extracellular loop (EL1) of Ste2p, the Saccharomyces cerevisiae alpha-factor mating receptor, were activated using these reagents and the efficiencies of activation and rates of heterodimerization were analyzed. Activation of NT peptides with DNTP and 4-PDS resulted in about 60% yield, but heterodimerization was rapid and nearly quantitative. Double transmembrane domain protein fragments were biosynthesized and used in Guided Reconstitution reactions. A 102-residue fragment, 2TM-tail (Ste2p(G31-I120C)), was heterodimerized with CT-EL1-tailDTNP at pH 4.6 with a yield of ~75%. A 132-residue fragment, 2TMlong-tail [Ste2p(M1-I120C)], was expressed in both unlabeled and 15N-labeled forms and used, with a peptide comprising the third transmembrane domain, to generate a 180-residue segmentally-labeled 3TM protein that was found to be segmentally labelled using [15N,1H]-HSQC analysis. Our data indicate that the Guided Reconstitution method would be applicable to the segmental labeling of a membrane protein with 3 transmembrane domains and may prove useful in the preparation of an intact reconstituted GPCR for use in biophysical analysis and structure determination.
The structural characterization of G protein-coupled receptors has surged since the development of methodologies to facilitate the crystallization of these highly helical, seven transmembrane, integral membrane receptors. In the past seven years, eighteen GPCR structures were determined by X-ray crystallography. The crystal structures represent a static picture of these conformationally flexible signal transducers. Analyses that probe their dynamics and conformational changes require other techniques, in particular solution state nuclear magnetic resonance studies. Such investigations are challenged by the size of GPCRs, their α-helical structure, which limits resonance dispersion, their tendencies to aggregate in micellar preparations and their conformational heterogeneity. For many years, groups have been studying GPCR fragments as a means to overcome some of these difficulties. The results of these fragment analyses are presented here. Review of the literature reveals that much of the original work depended on circular dichroism, infra-red spectroscopy and fluorescence approaches. High resolution structures obtained by NMR are compared, where applicable, to the available crystal structures. In most cases, the work done on fragments by biophysical analysis is validated by these comparisons. Our perspective on the field of GPCR fragment analysis is presented together with the future goals that must be considered if work with fragments is continued.
Folding of G-protein coupled receptors (GPCRs) according to the two-stage model (Popot et al., Biochemistry 29(1990), 4031) is postulated to proceed in 2 steps: Partitioning of the polypeptide into the membrane followed by diffusion until native contacts are formed. Herein we investigate conformational preferences of fragments of the yeast Ste2p receptor using NMR. Constructs comprising the first, the first two and the first three transmembrane (TM) segments, as well as a construct comprising TM1-TM2 covalently linked to TM7 were examined. We observed that the isolated TM1 does not form a stable helix nor does it integrate well into the micelle. TM1 is significantly stabilized upon interaction with TM2, forming a helical hairpin reported previously (Neumoin et al., Biophys. J. 96(2009), 3187), and in this case the protein integrates into the hydrophobic interior of the micelle. TM123 displays a strong tendency to oligomerize, but hydrogen exchange data reveal that the center of TM3 is solvent exposed. In all GPCRs so-far structurally characterized TM7 forms many contacts with TM1 and TM2. In our study TM127 integrates well into the hydrophobic environment, but TM7 does not stably pack against the remaining helices. Topology mapping in microsomal membranes also indicates that TM1 does not integrate in a membrane-spanning fashion, but that TM12, TM123 and TM127 adopt predominantly nativelike topologies. The data from our study would be consistent with the retention of individual helices of incompletely synthesized GPCRs in the vicinity of the translocon until the complete receptor is released into the membrane interior. (7), and the structure of a GPCRarrestin complex was solved (8).While our knowledge of the structure of GPCRs in various states and their mode of activation and desensitization is rapidly increasing, detailed information on their folding pathways is still lacking. The popular refined two-stage model from Popot and Engelman postulates that secondary structure forms when the peptide chain partitions into the membranewater interface (9-11). However, proteins destined for membrane insertion are generally subjected to the concerted action of translating ribosomes in the cytoplasm and translocon complexes located in the endoplasmic reticulum (ER) of eukaryotes or in the plasma membrane of bacteria (12). In order to traffic proteins to membranes in most cells the signal-recognition particle targets the nascent chains emerging from the ribosome tunnel to the translocon complex (13 (12,(18)(19)(20). Folding of polytopic membrane proteins is the result of a series of events that include helixinsertion into the hydrophobic core and sequestering of loop-sequences into cytosolic or extracellular space (11,21). The timing of the chain insertion and the localization of TM helices would be expected to be a consequence of the amino acid sequence and the interaction of the growing polypeptide chain with the membrane. Recently, however, the first evidence was obtained that helices might change their locat...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.