Invited review GPCR structural characterization: Using fragments as building blocks to determine a complete structure

Comparison of Experimental Approaches Used to Determine the Structure and Function of the Class D G Protein-Coupled Yeast α-Factor Receptor

The Saccharomyces cerevisiae α-factor mating pheromone receptor (Ste2p) has been studied as a model for the large medically important family of G protein-coupled receptors. Diverse yeast genetic screens and high-throughput mutagenesis of STE2 identified a large number of loss-of-function, constitutively-active, dominant-negative, and intragenic second-site suppressor mutants as well as mutations that specifically affect pheromone binding. Facile genetic manipulation of Ste2p also aided in targeted biochemical approaches, such as probing the aqueous accessibility of substituted cysteine residues in order to identify the boundaries of the seven transmembrane segments, and the use of cysteine disulfide crosslinking to identify sites of intramolecular contacts in the transmembrane helix bundle of Ste2p and sites of contacts between the monomers in a Ste2p dimer. Recent publication of a series of high-resolution cryo-EM structures of Ste2p in ligand-free, agonist-bound and antagonist-bound states now makes it possible to evaluate the results of these genetic and biochemical strategies, in comparison to three-dimensional structures showing activation-related conformational changes. The results indicate that the genetic and biochemical strategies were generally effective, and provide guidance as to how best to apply these experimental strategies to other proteins. These strategies continue to be useful in defining mechanisms of signal transduction in the context of the available structures and suggest aspects of receptor function beyond what can be discerned from the available structures.

A Paradigm for Peptide Hormone-GPCR Analyses

Work from our laboratories over the last 35 years that has focused on Ste2p, a G protein-coupled receptor (GPCR), and its tridecapeptide ligand α-factor is reviewed. Our work utilized the yeast Saccharomyces cerevisiae as a model system for understanding peptide-GPCR interactions. It explored the structure and function of synthetic α-factor analogs and biosynthetic receptor domains, as well as designed mutations of Ste2p. The results and conclusions are described using the nuclear magnetic resonance interrogation of synthetic Ste2p transmembrane domains (TMs), the fluorescence interrogation of agonist and antagonist binding, the biochemical crosslinking of peptide analogs to Ste2p, and the phenotypes of receptor mutants. We identified the ligand-binding domain in Ste2p, the functional assemblies of TMs, unexpected and interesting ligand analogs; gained insights into the bound α-factor structure; and unraveled the function and structures of various Ste2p domains, including the N-terminus, TMs, loops connecting the TMs, and the C-terminus. Our studies showed interactions between specific residues of Ste2p in an active state, but not resting state, and the effect of ligand activation on the dimerization of Ste2p. We show that, using a battery of different biochemical and genetic approaches, deep insight can be gained into the structure and conformational dynamics of GPCR-peptide interactions in the absence of a crystal structure.

Understanding GPCR Recognition and Folding from NMR Studies of Fragments

Cotranslational protein folding is a vectorial process, and for membrane proteins, N-terminal helical segments are the first that become available for membrane insertion. While structures of many G-protein coupled receptors (GPCRs) in various states have been determined, the details of their folding pathways are largely unknown. The seven transmembrane (TM) helices of GPCRs often contain polar residues within the hydrophobic core, and some of the helices in isolation are predicted to be only marginally stable in a membrane environment. Here we review our efforts to describe how marginally hydrophobic TM helices of GPCRs integrate into the membrane in the absence of all compensating interhelical contacts, ideally capturing early biogenesis events. To this end, we use truncated GPCRs, here referred to as fragments. We present data from the human Y4 and the yeast Ste2p receptors in detergent micelles derived from solution NMR techniques. We find that the secondary structure in the fragments is similar to corresponding parts of the entire receptors. However, uncompensated polar or charged residues destabilize the helices, and prevent proper integration into the lipid bilayer, in agreement with the biophysical scales from Wimley and White for the partitioning of amino acids into the membrane-interior. We observe that the stability and integration of single TM helices is improved by adding neighboring helices. We describe a topology study, in which all possible forms of the Y4 receptor were made so that the entire receptor is truncated from the N-terminus by one TM helix at a time. We discover that proteins with an increasing number of helices assume a more defined topology. In a parallel study, we focused on the role of extracellular loops in ligand recognition. We demonstrate that transferring all loops of the human Y1 receptor onto the E. coli outer membrane protein OmpA in a suitable topology results in a chimeric receptor that displays, albeit reduced, affinity and specificity for the cognate ligand. Our data indicate that not all TM helices will spontaneously insert into the helix, and we suggest that at least for some GPCRs, N-terminal segments might remain associated with the translocon until their interacting partners are biosynthesized.

Cotranslational protein folding is a vectorial process, and for membrane proteins, N-terminal helical segments are the first that become available for membrane insertion. Here fragments corresponding to these segments are investigated by NMR.

Doxazosin attenuates renal matrix remodeling mediated by anti-α1-adrenergic receptor antibody in a rat model of diabetes mellitus

Diabetic nephropathy is a major complication of diabetes mellitus (DM). Recent studies suggest that immunological mechanisms have a key role in the pathogenesis of DM, therefore these mechanisms may be important targets for diabetes therapy. The present study evaluated the effects of anti-α₁-adrenergic receptor antibody (α₁-R Ab) mediation and doxazosin treatment in a rat model of DM. It was observed that levels of 24-h urinary protein, serum creatinine and transforming growth factor-β₁ in DM were significantly increased after α₁-R Ab mediation (all P<0.05). In addition, electron microscopy identified severe damage in the renal tissue microstructures of DM rats following α₁-R Ab mediation, while only mild abnormalities were observed in that of healthy rats mediated with α₁-R Ab and of untreated DM rats. No marked abnormalities were observed in the renal tissue of healthy blank controls. Furthermore, in DM rats treated with α₁-R Ab mediation + doxazosin intervention, the expression of TGF-β₁ significantly decreased, and renal functions and renal matrix remodeling were significantly improved, relative to untreated DM controls (P<0.01). These results suggest that α₁-R Ab may be involved in renal matrix remodeling during DM, and that kidney protection during DM may be achieved through treatment with corresponding receptor antagonists.

Small expression tags enhance bacterial expression of the first three transmembrane segments of the apelin receptor

G-protein coupled receptors (GPCRs) are inherently dynamic membrane protein modulators of various important cellular signaling cascades. The apelin receptor (AR or APJ) is a class A GPCR involved in numerous physiological processes, implicated in angiogenesis during tumour formation and as a CD4 co-receptor for entry of human immunodeficiency virus type 1 (HIV-1) to cells. Due to the lack of efficient methods to produce full-length GPCRs enriched with nuclear magnetic resonance (NMR) active (15)N, (13)C, and (or) (2)H isotopes, small GPCR fragments typically comprising 1-2 transmembrane segments are frequently studied using NMR spectroscopy. Here, we report successful overexpression of transmembrane segments 1-3 of AR (AR_TM1-3) in the C41(DE3) strain of Escherichia coli using an AT-rich gene tag previously reported to enhance cell-free expression yields. The resulting protein, with 6 additional N-terminal residues due to the expression tag, was purified using high-performance liquid chromatography (HPLC). Far UV circular dichroism spectropolarimetry demonstrates that AR_TM1-3 has the predicted ~40% α-helical character in membrane-mimetic environments. (1)H-(15)N HSQC NMR experiments imply amenability to high-resolution NMR structural characterization and stability in solution for weeks. Notably, this small expression tag approach may also be generally applicable to other membrane proteins that are difficult to express in E. coli.