A Profile of the Residues in the First Intracellular Loop Critical for Gs-Mediated Signaling of Human Prostacyclin Receptor Characterized by an Integrative Approach of NMR-Experiment and Mutagenesis

Structural features of the G-protein/GPCR interactions

BACKGROUND

The details of the functional interaction between G proteins and the G protein coupled receptors (GPCRs) have long been subjected to extensive investigations with structural and functional assays and a large number of computational studies.

SCOPE OF REVIEW

The nature and sites of interaction in the G-protein/GPCR complexes, and the specificities of these interactions selecting coupling partners among the large number of families of GPCRs and G protein forms, are still poorly defined.

MAJOR CONCLUSIONS

Many of the contact sites between the two proteins in specific complexes have been identified, but the three dimensional molecular architecture of a receptor-Gα interface is only known for one pair. Consequently, many fundamental questions regarding this macromolecular assembly and its mechanism remain unanswered.

GENERAL SIGNIFICANCE

In the context of current structural data we review the structural details of the interfaces and recognition sites in complexes of sub-family A GPCRs with cognate G-proteins, with special emphasis on the consequences of activation on GPCR structure, the prevalence of preassembled GPCR/G-protein complexes, the key structural determinants for selective coupling and the possible involvement of GPCR oligomerization in this process.

Signal protein-derived peptides as functional probes and regulators of intracellular signaling

The functionally important regions of signal proteins participating in their specific interaction and responsible for transduction of hormonal signal into cell are rather short in length, having, as a rule, 8 to 20 amino acid residues. Synthetic peptides corresponding to these regions are able to mimic the activated form of full-size signal protein and to trigger signaling cascades in the absence of hormonal stimulus. They modulate protein-protein interaction and influence the activity of signal proteins followed by changes in their regulatory and catalytic sites. The present review is devoted to the achievements and perspectives of the study of signal protein-derived peptides and to their application as selective and effective regulators of hormonal signaling systems in vitro and in vivo. Attention is focused on the structure, biological activity, and molecular mechanisms of action of peptides, derivatives of the receptors, G protein α subunits, and the enzymes generating second messengers.

The human prostacyclin receptor from structure function to disease

Thirty years have passed since Vane and colleagues first described a substance, prostanoid X, from microsomal fractions (later called prostacyclin) that relaxed rather than contracted mesenteric arteries. The critical role of prostacyclin in many pathophysiological conditions, such as atherothrombosis, has only recently become appreciated (through receptor knockout mice studies, selective cyclooxygenase-2 inhibition clinical trials, and the discovery of dysfunctional prostacyclin receptor genetic variants). Additionally, important roles in such diverse areas as pain and inflammation, and parturition are being uncovered. Prostacyclin-based therapies, currently used for pulmonary hypertension, are accordingly emerging as possible treatments for such diseases, fueling interests in structure function studies for the receptor and signal transduction pathways in native cells. The coming decade is likely to yield many further exciting advances.

Ligand-specific conformation determines agonist activation and antagonist blockade in purified human thromboxane A2 receptor

The binding of an agonist to a G protein-coupled receptor (GPCR) causes its coupling to different G proteins, which mediate signaling. However, the binding of an antagonist to the same site of the GPCR could not induce coupling. To understand the molecular mechanism involved, the structural flexibility of the purified human thromboxane A2 receptor (TP) was characterized by spectroscopic approaches, while bound to an agonist or antagonist. Circular dichroism not only revealed that the purified TP adopted more than 50% helical conformation in solution but also showed that the antagonist, SQ29,548, could induce more of a beta-sheet structure in the TP than that of the agonist, U46619. Also, fluorescence studies showed that the antagonist induced the intrinsic Trp fluorescence signal change more than the agonist. Furthermore, three of the nine tryptophan residues involved in the different ligand-based structural changes were demonstrated by NMR spectroscopy. Low pH-induced changes in the receptor conformation and molecular interaction field dramatically increased the agonist binding but did not significantly affect the antagonist binding. Different conformational changes were also observed in the TP reconstituted into phosphatidylcholine/phosphatidylserine/phosphatydylethanolamine-formed liposomes. These studies are the first to show a possible mechanism of the ligand-specific conformation-dependent agonist activation and antagonist blockage in the GPCR.

A profile of the residues in the second extracellular loop that are critical for ligand recognition of human prostacyclin receptor

The residues in the second extracellular loop (eLP2) of the prostanoid receptors, which are important for specific ligand recognition, were previously predicted in our earlier studies of the thromboxane A2 receptor (TP) using a combination of NMR spectroscopy and recombinant protein approaches. To further test this hypothesis, another prostanoid receptor, the prostacyclin receptor (IP), which has opposite biological characteristics to that of TP, was used as a model for these studies. A set of recombinant human IPs with site-directed mutations at the nonconserved eLP2 residues were constructed using an Ala-scanning approach, and then expressed in HEK293 and COS-7 cells. The expression levels of the recombinant receptors were six-fold higher in HEK293 cells than in COS-7 cells. The residues important for ligand recognition and binding within the N-terminal segment (G159, Q162, and C165) and the C-terminal segment (L172, R173, M174, and P179) of IP eLP2 were identified by mutagenesis analyses. The molecular mechanisms for the specific ligand recognition of IP were further demonstrated by specific site-directed mutagenesis using different amino acid residues with unique chemical properties for the key residues Q162, L172, R173, and M174. A comparison with the corresponding functional residues identified in TP eLP2 revealed that three (Q162, R173, and M174) of the four residues are nonconserved, and these are proposed to be involved in specific ligand recognition. We discuss the importance of G159 and P179 in ligand recognition through configuration of the loop conformation is discussed. These studies have further indicated that characterization of the residues in the eLP2 regions for all eight prostanoid receptors could be an effective approach for uncovering the molecular mechanisms of the ligand selectivities of the G-protein-coupled receptors.

Assembling NMR structures for the intracellular loops of the human thromboxane A2 receptor: implication of the G protein-coupling pocket

It has been reported that the multiple intracellular loops (iLPs) of the thromboxane A(2) receptor (TP) are involved in the receptor G protein coupling. In this study, a high-resolution 2D NMR technique was used to determine the 3D structures of the first, second, and third iLPs of the TP using synthetic peptides constrained into the loop structures. 2D (1)H NMR spectra, TOCSY and NOESY were obtained for the two peptides from proton NMR experiments. The NMR data was processed and assigned through the Felix 2000 program. Standard methods were used to acquire sequence-specific assignments. Structure calculations were processed through DGII and NMR refinement programs within the Insight II program. We were able to calculate and use the NOE constraints to obtain the superimposed structure of 10 structures for each iLP peptide. The NMR-determined structures of the iLP peptides were used to refine a homology model of the TP. A 3D G-protein-binding cavity, formed by the three intracellular loops, was predicted by the docking of the C-terminal domain of the Galphaq. Based on the structural model and the previous mutagenesis studies, the residues, R130, R60, C223, F138, L360, V361, E358 and Y359, which are important for interaction with the G protein, were further highlighted. These results reveal the possibly important molecular mechanisms in TP signaling and provide structural information to characterize other prostanoid receptor signalings.

Human prostacyclin receptor structure and function from naturally-occurring and synthetic mutations

Prostacyclin (PGI2) is released by vascular endothelial cells and serves as a potent vasodilator, inhibitor of platelet aggregation (anti-thrombotic), and moderator of vascular smooth muscle cell proliferation-migration-differentiation (anti-atherosclerotic). These actions are mediated via a seven transmembrane-spanning G-protein coupled receptor (GPCR), known as the human prostacyclin receptor or hIP. Animal studies using prostacyclin receptor knock-out (IP-/-) mice have revealed increased propensities towards thrombosis, intimal hyperplasia, atherosclerosis, restenosis, as well as reperfusion injury. Of further importance has been the world-wide withdrawal of selective COX-2 inhibitors, due to their discriminating suppression of COX-2-derived PGI2 and its cardioprotective effects, leading to increased cardiovascular events, including myocardial infarction and thrombotic stroke. Over the last decade, mutagenesis studies of the IP receptor, in conjunction with in vitro functional assays and molecular modeling, have provided critical insights into the molecular mechanisms of both agonist binding and receptor activation. Most recently, the discovery of naturally-occurring and dysfunctional mutations within the hIP has provided additional insights into the proposed cardioprotective role of prostacyclin. The aim of this review is to summarize the most recent findings regarding hIP receptor structure-function that have developed through the study of both synthetic and naturally-occurring mutations.