Selective polypeptide ligand binding to the extracellular surface of the transmembrane domains of the class B GPCRs GLP-1R and GCGR.

Crystal and cryo-EM structures of the glucagon-like peptide-1 receptor (GLP-1R) and glucagon receptor (GCGR) bound with their peptide ligands have been obtained with full-length constructs, indicating that the extracellular domain (ECD) is indispensable for specific ligand binding. This article complements these data with studies of ligand recognition of the two receptors in solution. Paramagnetic NMR relaxation enhancement measurements using dual labeling with fluorine-19 probes on the receptor and nitroxide spin labels on the peptide ligands provided new insights. The glucagon-like peptide-1 (GLP-1) was found to interact with GLP-1R by selective binding to the extracellular surface. The ligand selectivity toward the extracellular surface of the receptor was preserved in the transmembrane domain (TMD) devoid of the ECD. The dual labeling approach further provided evidence of cross-reactivity of GLP-1R and GCGR with glucagon and GLP-1, respectively, which is of interest in the context of medical treatments using combinations of the two polypeptides.

Structural basis for the DNA-binding activity of human ARID4B Tudor domain.

Human ARID4A and ARID4B are homologous proteins that are important in controlling gene expression and epigenetic regulation but have distinct functions. Previous studies have shown that the N-terminal domain of ARID4A is an unusual interdigitated double Tudor domain with DNA-binding activity. However, how the Tudor domain of ARID4B differs from that of ARID4A remains unknown. Here, we found that the ARID4B Tudor domain has significantly weaker DNA affinity than the ARID4A Tudor domain despite sharing more than 80% sequence identity. Structure determination and DNA titration analysis indicated that the ARID4B Tudor domain is also an interdigitated double Tudor domain with a DNA-binding surface similar to ARID4A. We identified a residue close to the DNA-binding site of the Tudor domain that differs between ARID4A and ARID4B. The Leu50 in ARID4A is Glu50 in ARID4B, and the latter forms salt bridges with two lysine residues at the DNA-binding surface. This causes a decrease in the strength of positive charge, thus reducing DNA-binding affinity while significantly increasing protein stability. We also found that a C-terminal extension region enhances the DNA-binding affinity of the ARID4B Tudor domain. This C-terminal extension is disordered and contains a positively charged RGR motif, providing an additional DNA-binding site. Finally, sequence and phylogenetic analyses indicated that the residue differences and the presence of the RGR extension region are conserved. These results provide new insight into the functional differences between ARID4A and ARID4B proteins, as well as elucidating the function of the disordered regions in these proteins.

Design and preparation of the class B G protein-coupled receptors GLP-1R and GCGR for 19 F-NMR studies in solution.

The human glucagon-like peptide-1 receptor (GLP-1R) and the glucagon receptor (GCGR) are class B G protein-coupled receptors (GPCRs) that are activated by interactions with, respectively, the glucagon-like peptide-1 (GLP-1) and glucagon (GCG). These polypeptide hormones are involved in the regulation of lipid and cholic acid metabolism, and thus play an important role in the pathogenesis of glucose metabolism and diabetes mellitus, which attracts keen interest of these GPCRs as drug targets. GLP-1R and GCGR have therefore been extensively investigated by X-ray crystallography and cryo-electron microscopy (cryo-EM), so that their structures are well known. Here, we present the groundwork for using nuclear magnetic resonance (NMR) spectroscopy in solution to complement the molecular architectures with information on intramolecular dynamics and on the thermodynamics and kinetics of interactions with physiological ligands and extrinsic drug candidates. This includes the generation of novel, near-wild-type constructs of GLP-1R and GCGR, optimization of the solution conditions for NMR studies in detergent micelles and in nanodiscs, post-translational chemical introduction of fluorine-19 NMR probes, and sequence-specific assignments of the 19 F-labels attached to indigenous cysteines. Addition of the negative allosteric modulator (NAM) NNC0640 was critically important for obtaining the long-time stability needed for our NMR experiments, and we report on novel insights into the allosteric effects arising from binding of NNC0640 to the transmembrane domain of GLP-1R (GLP-1R[TMD]).

S-Glutathionylation of human inducible Hsp70 reveals a regulatory mechanism involving the C-terminal α-helical lid.

Heat shock protein 70 (Hsp70) proteins are a family of ancient and conserved chaperones. Cysteine modifications have been widely detected among different Hsp70 family members in vivo, but their effects on Hsp70 structure and function are unclear. Here, we treated HeLa cells with diamide, which typically induces disulfide bond formation except in the presence of excess GSH, when glutathionylated cysteines predominate. We show that in these cells, HspA1A (hHsp70) undergoes reversible cysteine modifications, including glutathionylation, potentially at all five cysteine residues. In vitro experiments revealed that modification of cysteines in the nucleotide-binding domain of hHsp70 is prevented by nucleotide binding but that Cys-574 and Cys-603, located in the C-terminal α-helical lid of the substrate-binding domain, can undergo glutathionylation in both the presence and absence of nucleotide. We found that glutathionylation of these cysteine residues results in unfolding of the α-helical lid structure. The unfolded region mimics substrate by binding to and blocking the substrate-binding site, thereby promoting intrinsic ATPase activity and competing with binding of external substrates, including heat shock transcription factor 1 (Hsf1). Thus, post-translational modification can alter the structure and regulate the function of hHsp70.

Trimethylsilyl reporter groups for NMR studies of conformational changes in G protein-coupled receptors.

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.

The C-terminal GGAP motif of Hsp70 mediates substrate recognition and stress response in yeast.

The allosteric coupling of the highly conserved nucleotide- and substrate-binding domains of Hsp70 has been studied intensively. In contrast, the role of the disordered, highly variable C-terminal region of Hsp70 remains unclear. In many eukaryotic Hsp70s, the extreme C-terminal EEVD motif binds to the tetratricopeptide-repeat domains of Hsp70 co-chaperones. Here, we discovered that the TVEEVD sequence of Saccharomyces cerevisiae cytoplasmic Hsp70 (Ssa1) functions as a SUMO-interacting motif. A second C-terminal motif of ∼15 amino acids between the α-helical lid and the extreme C terminus, previously identified in bacterial and eukaryotic organellar Hsp70s, is known to enhance chaperone function by transiently interacting with folding clients. Using structural analysis, interaction studies, fibril formation assays, and in vivo functional assays, we investigated the individual contributions of the α-helical bundle and the C-terminal disordered region of Ssa1 in the inhibition of fibril formation of the prion protein Ure2. Our results revealed that although the α-helical bundle of the Ssa1 substrate-binding domain (SBDα) does not directly bind to Ure2, the SBDα enhances the ability of Hsp70 to inhibit fibril formation. We found that a 20-residue C-terminal motif in Ssa1, containing GGAP and GGAP-like tetrapeptide repeats, can directly bind to Ure2, the Hsp40 co-chaperone Ydj1, and α-synuclein, but not to the SUMO-like protein SMT3 or BSA. Deletion or substitution of the Ssa1 GGAP motif impaired yeast cell tolerance to temperature and cell-wall damage stress. This study highlights that the C-terminal GGAP motif of Hsp70 is important for substrate recognition and mediation of the heat shock response.

Selection of an ASIC1a-blocking combinatorial antibody that protects cells from ischemic death.

Acid-sensing ion channels (ASICs) have emerged as important, albeit challenging therapeutic targets for pain, stroke, etc. One approach to developing therapeutic agents could involve the generation of functional antibodies against these channels. To select such antibodies, we used channels assembled in nanodiscs, such that the target ASIC1a has a configuration as close as possible to its natural state in the plasma membrane. This methodology allowed selection of functional antibodies that inhibit acid-induced opening of the channel in a dose-dependent way. In addition to regulation of pH, these antibodies block the transport of cations, including calcium, thereby preventing acid-induced cell death in vitro and in vivo. As proof of concept for the use of these antibodies to modulate ion channels in vivo, we showed that they potently protect brain cells from death after an ischemic stroke. Thus, the methodology described here should be general, thereby allowing selection of antibodies to other important ASICs, such as those involved in pain, neurodegeneration, and other conditions.

Solvent-accessibility of discrete residue positions in the polypeptide hormone glucagon by 19F-NMR observation of 4-fluorophenylalanine.

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).

Resonance assignments for the substrate binding domain of Hsp70 chaperone Ssa1 from Saccharomyces cerevisiae.

Hsp70 chaperone proteins play crucial roles in the cell. Extensive structural and functional studies have been performed for bacterial and mammalian Hsp70s. Ssa1 from Saccharomyces cerevisiae is a member of the Hsp70 family. In vivo and biochemical studies on Ssa1 have revealed that it regulates prion propagation and the cell cycle. However, no structural data has been obtained for Ssa1 up to now. Here we report the almost complete (96 %) (1)H, (13)C, (15)N backbone and side chain NMR assignment of the 18.8 kDa Ssa1 substrate binding domain. The construct includes residues 382-554, which corresponds to the entire substrate binding domain and two following α-helices in homologous structures. The secondary structure predicted from the assigned chemical shifts is consistent with that of homologous Hsp70 substrate binding domains.

Influence of specific HSP70 domains on fibril formation of the yeast prion protein Ure2.

Ure2p is the protein determinant of the Saccharomyces cerevisiae prion state [URE3]. Constitutive overexpression of the HSP70 family member SSA1 cures cells of [URE3]. Here, we show that Ssa1p increases the lag time of Ure2p fibril formation in vitro in the presence or absence of nucleotide. The presence of the HSP40 co-chaperone Ydj1p has an additive effect on the inhibition of Ure2p fibril formation, whereas the Ydj1p H34Q mutant shows reduced inhibition alone and in combination with Ssa1p. In order to investigate the structural basis of these effects, we constructed and tested an Ssa1p mutant lacking the ATPase domain, as well as a series of C-terminal truncation mutants. The results indicate that Ssa1p can bind to Ure2p and delay fibril formation even in the absence of the ATPase domain, but interaction of Ure2p with the substrate-binding domain is strongly influenced by the C-terminal lid region. Dynamic light scattering, quartz crystal microbalance assays, pull-down assays and kinetic analysis indicate that Ssa1p interacts with both native Ure2p and fibril seeds, and reduces the rate of Ure2p fibril elongation in a concentration-dependent manner. These results provide new insights into the structural and mechanistic basis for inhibition of Ure2p fibril formation by Ssa1p and Ydj1p.