Effects of phosphorylation on the structure of the G-protein receptor rhodopsin

Phosphorylation-induced conformational changes of photoactivated rhodopsin probed by fluorescent labeling at Cys140 and Cys316

In order to monitor conformational changes following photoactivation and phosphorylation of bovine rhodopsin, the two reactive sulfhydryl groups at Cys¹⁴⁰ and Cys³¹⁶ were specifically labeled with the monobromobimane (mBBr) fluorophore. Although alterations in conformation after light exposure of rhodopsin were not detected by fluorescence excitation scans (300-450 nm) of the mBBr-labeled protein, the fluorescence signal was reduced ∼ 90% in samples containing photoactivated phosphorhodopsin. Predominant labeling at either Cys¹⁴⁰ or Cys³¹⁶ in light-activated and phosphorylated rhodopsin merely generated a decrease of ∼38% and 28%, respectively, in the fluorescence excitation intensity. Thus, neither mBBr-modified Cys¹⁴⁰ nor mBBr-modified Cys³¹⁶ were involved single-handedly in the remarkable fall seen on the signal following phosphorylation of the protein; rather, the incorporation of phosphate groups on the mBBr-labeled light-activated rhodopsin appeared to affect its fluorescence signal in a cooperative or synergistic manner. These findings demonstrated that the phosphorylation of specific hydroxyl groups at the carboxyl terminal tail of rhodopsin causes definite conformational changes in the three-dimensional fold of the protein. Apparently, amino acid residues that are buried in the interior of the inactive protein become accessible following illumination and phosphorylation of rhodopsin, quenching in turn the fluorescence excitation signal of mBBr-modified rhodopsin.

Unraveling the structure and function of G protein-coupled receptors through NMR spectroscopy

G protein-coupled receptors (GPCRs) are a large superfamily of signaling proteins expressed on the plasma membrane. They are involved in a wide range of physiological processes and, therefore, are exploited as drug targets in a multitude of therapeutic areas. In this extent, knowledge of structural and functional properties of GPCRs may greatly facilitate rational design of modulator compounds. Solution and solid-state nuclear magnetic resonance (NMR) spectroscopy represents a powerful method to gather atomistic insights into protein structure and dynamics. In spite of the difficulties inherent the solution of the structure of membrane proteins through NMR, these methods have been successfully applied, sometimes in combination with molecular modeling, to the determination of the structure of GPCR fragments, the mapping of receptor-ligand interactions, and the study of the conformational changes associated with the activation of the receptors. In this review, we provide a summary of the NMR contributions to the study of the structure and function of GPCRs, also in light of the published crystal structures.

Membrane protein fragments reveal both secondary and tertiary structure of membrane proteins

Structural data on membrane proteins, while crucial to understanding cellular function, are scarce due to difficulties in applying to membrane proteins the common techniques of structural biology. Fragments of membrane proteins have been shown to reflect, in many cases, the secondary structure of the parent protein with fidelity and are more amenable to study. This chapter provides many examples of how the study of membrane protein fragments has provided new insight into the structure of the parent membrane protein.

Conformational changes in the phosphorylated C-terminal domain of rhodopsin during rhodopsin arrestin interactions

Phosphorylation of activated G-protein-coupled receptors and the subsequent binding of arrestin mark major molecular events of homologous desensitization. In the visual system, interactions between arrestin and the phosphorylated rhodopsin are pivotal for proper termination of visual signals. By using high resolution proton nuclear magnetic resonance spectroscopy of the phosphorylated C terminus of rhodopsin, represented by a synthetic 7-phosphopolypeptide, we show that the arrestin-bound conformation is a well ordered helix-loop structure connected to rhodopsin via a flexible linker. In a model of the rhodopsin-arrestin complex, the phosphates point in the direction of arrestin and form a continuous negatively charged surface, which is stabilized by a number of positively charged lysine and arginine residues of arrestin. Opposite to the mostly extended structure of the unphosphorylated C-terminal domain of rhodopsin, the arrestin-bound C-terminal helix is a compact domain that occupies a central position between the cytoplasmic loops and occludes the key binding sites of transducin. In conjunction with other binding sites, the helix-loop structure provides a mechanism of shielding phosphates in the center of the rhodopsin-arrestin complex and appears critical in guiding arrestin for high affinity binding with rhodopsin.

The arrestin-bound conformation and dynamics of the phosphorylated carboxy-terminal region of rhodopsin

Visual arrestin binds to the phosphorylated carboxy-terminal region of rhodopsin to block interactions with transducin and terminate signaling in the rod photoreceptor cells. A synthetic seven-phospho-peptide from the C-terminal region of rhodopsin, Rh(330-348), has been shown to bind arrestin and mimic inhibition of signal transduction. In this study, we examine conformational changes in this synthetic peptide upon binding to arrestin by high-resolution proton nuclear magnetic resonance (NMR). We show that the peptide is completely disordered in solution, but becomes structured upon binding to arrestin. A control, unphosphorylated peptide that fails to bind to arrestin remains highly disordered. Specific NMR distance constraints are used to model the arrestin-bound conformation. The models suggest that the phosphorylated carboxy-terminal region of rhodopsin, Rh(330-348), undergoes significant conformational changes and becomes structured upon binding to arrestin.

Conformational flexibility of three cytoplasmic segments of the angiotensin II AT1A receptor: a circular dichroism and fluorescence spectroscopy study

The conformation of three synthetic peptides encompassing the proximal and distal half of the third intracellular loop (Ni3 and Ci3) and a portion of the cytoplasmic tail (fCT) of the angiotensin II AT1A receptor has been studied using circular dischroism and fluorescence spectroscopies. The results show that the conformation of the peptides is modulated in various ways by the environmental conditions (pH, ionic strength and dielectric constant). Indeed, Ni3 and fCT fold into helical structures that possess distinct stability and polarity due to the diverse forces involved: mainly polar interactions in the first case and a combination of polar and hydrophobic interactions in the second. The presence of these various features also produce distinct intermolecular interactions. Ci3, instead, exists as an ensemble of partially folded states in equilibrium. Since the corresponding regions of the angiotensin II AT1A receptor are known to play an important role in the receptor function, due to their ability to undergo conformational changes, these data provide some new clues about their different conformational plasticity.

Peptide hormone binding to G-protein-coupled receptors: structural characterization via NMR techniques

G-protein-coupled receptors (GPCRs) allow cells to respond to calcium, hormones, and neurotransmitters. Not surprisingly, they currently make up the largest family of validated drug targets. Rational drug design for molecular regulators targeting GPCRs has been limited to theoretical-based computational approaches. X-ray crystallography of intact GPCRs has provided the topological orientation of the seven transmembrane helices, but limited structural information of the extracellular and intracellular loops and protein termini. In this review we detail an NMR-based approach which provides the high-resolution structural features on the extracellular domains of GPCRs and the ligand/receptor complexes formed upon titration of the peptide hormone. The results provide important contact points and a high-resolution description of the ligand/receptor interactions, which may be useful for the rational design of therapeutic agents targeting GPCRs. Recent results from our investigation of the cholecystokinin peptide hormone system are used to highlight this approach.

Solution structure of the loops of bacteriorhodopsin closely resembles the crystal structure

Bacteriorhodopsin is one of very few transmembrane proteins for which high resolution structures have been solved. The structure shows a bundle of seven helices connected by six turns. Some turns in proteins are stabilized by short range interactions and can behave as small domains. These observations suggest that peptides containing the sequence of the turns in a membrane protein such as bacteriorhodopsin may form stable turn structures in solution. To test this hypothesis, we determined the solution structure of three peptides each containing the sequence of one of the turns in bacteriorhodopsin. The solution structures of the peptides closely resemble the structures of the corresponding turns in the high resolution structures of the intact protein.

Structures of the intradiskal loops and amino terminus of the G-protein receptor, rhodopsin

The intradiskal surface of the transmembrane protein, rhodopsin, consists of the amino terminal domain and three loops connecting six of the seven transmembrane helices. This surface corresponds to the extracellular surface of other G-protein receptors. Peptides that represent each of the extramembraneous domains on this surface (three loops and the amino terminus) were synthesized. These peptides also included residues which, based on a hydrophobic plot, could be expected to be part of the transmembrane helix. The structure of each of these peptides in solution was then determined using two-dimensional 1H nuclear magnetic resonance. All peptide domains showed ordered structures in solution. The structures of each of the peptides from intradiskal loops of rhodopsin exhibited a turn in the central region of the peptide. The ends of the peptides show an unwinding of the transmembrane helices to form this turn. The amino terminal domain peptide exhibited alpha-helical regions with breaks and bends at proline residues. This region forms a compact domain. Together, the structures for the loop and amino terminus domains indicate that the intradiskal surface of rhodopsin is ordered. These data further suggest a structural motif for short loops in transmembrane proteins. The ordered structures of these loops, in the absence of the transmembrane helices, indicate that the primary sequences of these loops are sufficient to code for the turn.

Human complement 5a (C5a) anaphylatoxin receptor (CD88) phosphorylation sites and their specific role in receptor phosphorylation and attenuation of G protein-mediated responses. Desensitization of C5a receptor controls superoxide production but not receptor sequestration in HL-60 cells

Upon agonist binding, the anaphylatoxin human complement 5a receptor (C5aR) has previously been found to be phosphorylated on the six serine residues of its carboxyl-terminal tail (Giannini, E., Brouchon, L., and Boulay, F. (1995) J. Biol. Chem. 270, 19166-19172). To evaluate the precise roles that specific phosphorylation sites may play in receptor signaling, a series of mutants were expressed transiently in COS-7 cells and stably in the physiologically relevant myeloid HL-60 cells. Ser(334) was found to be a key residue that controls receptor phosphorylation. Phosphorylation of either of two serine pairs, namely Ser(332) and Ser(334) or Ser(334) and Ser(338), was critical for the phosphorylation of C5aR and its subsequent desensitization. Full phosphorylation and desensitization of C5aR were obtained when these serines were replaced by aspartic acid residues. The mutation S338A had no marked effect on the agonist-mediated phosphorylation of C5aR, but it allowed a sustained C5a-evoked calcium mobilization in HL-60 cells. These findings and the ability of the S314A/S317A/S327A/S332A mutant receptor to undergo desensitization indicate that the phosphorylation of Ser(334) and Ser(338) is critical and sufficient for C5aR desensitization. The lack of phosphorylation was found to result not only in a sustained calcium mobilization and extracellular signal-regulated kinase 2 activity but also in the enhancement of the C5a-mediated respiratory burst in neutrophil-like HL-60 cells. For instance, the nonphosphorylatable S332A/S334A mutant receptor triggered a 1.8-2-fold higher production of superoxide as compared with the wild-type receptor. Interestingly, although the desensitization of this mutant was defective, it was sequestered with the same time course and the same efficiency as the wild-type receptor. Thus, in myeloid HL-60 cells, desensitization and sequestration of C5aR appear to occur through divergent molecular mechanisms.

Solution structure of the sixth transmembrane helix of the G-protein-coupled receptor, rhodopsin

Low resolution electron density maps have revealed the general orientation of the transmembrane helices of rhodopsin. However, high resolution structural information for the transmembrane domain of the G-protein-coupled receptor, rhodopsin, is as yet unavailable. In this study, a high resolution solution structure is reported for a 15 residue portion of the sixth transmembrane helix of rhodopsin (rhovih) as a free peptide. Helix 6 is one of the transmembrane helices of rhodopsin that contains a proline (amino acid residue 267) and the influence of this proline on the structure of this transmembrane domain was unknown. The structure obtained shows an alpha-helix through most of the sequence. The proline apparently induces only a modest distortion in the helix. Previously, the structure of the intradiskal loop connected to helix 6 was solved. The sequence of this loop contained five residues in common (residues 268-272) with the peptide reported here from the rhovih. The five residues in common between these two structures were superimposed to connect these two structures. The superposition showed a root mean square deviation of 0.2 A. Thus, this five residue sequence formed the same structure in both peptides, indicating that the structure of this region is governed primarily by short range interactions.