Signal transducer and activator of transcription 3 (Stat3) dimerization is commonly thought to be triggered by its tyrosine phosphorylation in response to interleukin-6 (IL-6) or other cytokines. Accumulating evidence from in vitro studies, however, suggests that cytoplasmic Stat3 may be associated with high-molecular-mass protein complexes and/or dimerize prior to its activation. To directly study Stat3 dimerization and subcellular localization upon cytokine stimulation, we used live-cell fluorescence spectroscopy and imaging microscopy combined with fluorescence resonance energy transfer (FRET). Stat3 fusion proteins with spectral variants of green fluorescent protein (GFP), cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) were constructed and expressed in human hepatoma cells (HepG2) and human embryonic kidney cells (HEK-293). Like wild-type Stat3, the fusion proteins redistributed from a preferentially cytoplasmic to nuclear localization upon IL-6 stimulation and supported IL-6-dependent target gene expression. FRET studies in cells co-expressing Stat3-CFP and Stat3-YFP demonstrated that Stat3 dimers exist in the absence of tyrosine phosphorylation. IL-6 induced a 2-fold increase of this basal FRET signal, indicating that tyrosine phosphorylation either increases the dimer/monomer ratio of Stat3 or induces a conformational change of the dimer yielding a higher FRET efficiency. Studies using a mutated Stat3 with a non-functional src-homology 2 (SH2) domain showed that the SH2 domain is essential for dimer formation of phosphorylated as well as non-phosphorylated Stat3. Furthermore, our data show that visualization of normalized FRET signals allow insights into the spatiotemporal dynamics of Stat3 signal transduction.

In contrast, cyclo S±S [Cys20,Cys24]pNPY was found to be a highly selective ligand at the Y 2 -receptor, binding only threefold less efficiently than NPY. Analogues containing variations of positions 31 and 32 showed highly reduced affinity to the Y 1 -receptor, while binding to the Y 5 -receptor was affected less. Inhibition of cAMP-accumulation of selected peptides with replacements within position 20±23 of NPY showed preserved agonistic properties. The NPY analogues tested give insights into ligand±receptor interaction of NPY at the Y 1 -, Y 2 -and Y 5 -receptor and contribute to our understanding of subtype selectivity. Furthermore, the Y 1 -receptor-preferring peptides are novel tools that will provide insight into the physiological role of the Y 1 -receptor.Keywords: food intake; neuropeptides; NPY; selective ligands; structure±affinity relationship.Obesity, food intake and energy homeostasis are key areas of growing importance because obesity is beginning to replace malnutrition and infectious diseases as the most significant contributor to ill health in the developed world [1]. Neuropeptide Y (NPY) is one of the most important effector molecules of leptin: it is a key molecule in the regulation of food intake, it acts via several different receptor subtypes and elicits several physiological effects. Profound effects on stimulation of food intake, secretion of luteinizing and growth hormone, and insulin release suggest an important role for NPY in the pathophysiology of obesity and diabetes [2±5]. A wide range of other effects of NPY have been reported, such as potent vasoconstriction [6], facilitation of learning and memory [7], modulation of locomotor behaviours [8], induction of hypothermia [9,10], inhibition of sexual behaviour [11], shifts in circadian rhythms [12], modulation of cardiorespiratory parameters[13], anxiolytic potency [14] and inhibition of alcohol consumption and resistance [15].NPY is a 36-amino acid peptide amide belonging to the family of pancreatic polypeptides that includes also pancreatic polypeptide (PP) and peptide YY; it was first isolated from pig brain in 1982 [16].NPY is widely distributed within the central nervous system and in the periphery. [25,26]. A putative Y 3 -receptor has been described only pharmacologically and no specific agonists or antagonists are known [27]. All receptors belong to the G-protein coupled receptor superfamily [28]. The correlation of a certain receptor subtype to a distinct physiological effect is not yet fully understood. Recent studies even suggest that different receptor subtypes redundantly mediate identical actions in a given system, as has been shown for the Y 1 -and Y 5 -receptor in the regulation of food intake [3]. Findings that deficiencies in either the Y 1 -or the Y 5 -receptors do not significantly impair the normal feeding response to fasting or NPY stimulation, strongly indicate that both Y receptors are involved in appetite regulation by NPY [29,30]. Selective receptor agonists or antagonists are useful tools with which to stu...

Up to now neuropeptide Y (NPY) receptors, which belong to the large family of G-protein-coupled receptors and are involved in a broad range of physiological processes, are believed to act as monomers. Studies with the Y 1 -receptor antagonist and Y 4 -receptor agonist GR231118, which binds with a 250-fold higher affinity than its monomer, led to the first speculation that NPY receptors can form homodimers. In the present work we used the fluorescence resonance energy transfer (FRET) to study homodimerization of the hY 1 -, hY 2 -, and hY 5 -receptors in living cells. For this purpose, we generated fusion proteins of NPY receptors and green fluorescent protein or spectral variants of green fluorescent protein (cyan, yellow, and red fluorescent protein), which can be used as FRET pairs. Two different FRET techniques, fluorescence microscopy and fluorescence spectroscopy, were applied. Both techniques clearly showed that the hY 1 -, hY 2 -, and hY 5 -NPY receptor subtypes are able to form homodimers. By using transiently transfected cells, as well as a stable cell line expressing the hY 2 -GFP fusion protein, we could demonstrate that the Y-GFP fusion proteins are still functional and that dimerization varies from 26 to 44% dependent on the receptor. However, homodimerization is influenced neither by NPY nor by G␣ protein binding. G-protein-coupled receptors (GPCRs)1 represent a superfamily of proteins characterized by seven transmembrane ␣-helices that interact with a family of heterotrimeric GTP-binding proteins, referred to as G-proteins (1). GPCRs are found in a wide range of organisms, and many kind of chemical messengers act through them, for example adrenalin, angiotensin, or neuropeptide Y (NPY). Ligands for GPCRs are involved in a broad range of physiological functions, and their malfunction is responsible for many diseases (2, 3).Until recently GPCRs were thought to function as monomers. However, a growing number of evidence suggests that they may exist as homodimers and heterodimers (4 -9). The existence of homodimers has been shown for several GPCRs including ␤ 2 -adrenergic receptor (10 -12), ␦-and -opioid receptors (6, 13), metabotropic glutamate receptor 5 (14), calcium-sensing receptor (15-17), m3 muscarinic receptor (18, 19), vasopressin V2-receptor (20), somatostatin (21, 22), and dopamine receptors (23)(24)(25). Whereas homodimerization of the somatostatin receptor 5 (21), the ␦-opioid receptor (13), and the ␤ 2 -adrenergic receptor (11) are agonist-mediated, dimerization of the -opioid receptor (26) is agonistindependent.So far photoaffinity labeling (27), cross-linking studies (15, 24), Western blot analysis (14), and immunoprecipitation (17,28,29) are the most frequently applied methods for the investigation of receptor homodimerization. Because of the development of new fluorescent dyes, novel fluorescent proteins, and new instrumentation, the fluorescence resonance energy transfer (FRET) obtained a renaissance (30) and could be applied recently for the investigation of receptor dimerization...

Neuropeptide Y (NPY) is one of the most abundant peptides in the central nervous system of mammalians. NPY acts by binding to at least five G-protein coupled receptors (GPCRs) which have been named Y1, Y2, Y4, Y5 and Y6. Three spin-labelled NPY analogues containing the nitroxide group of the amino acid TOAC (2.2.6.6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) as a paramagnetic probe were synthesized by solid-phase peptide synthesis. Synthetic problems owing to the sensitivity of nitroxide towards acidic and reducing conditions have been overcome by using a cleavage cocktail that contains anisole and cresol scavengers. Concerning the receptor binding preferences, the analogues [TOAC34]-pNPY and [Ala31, TOAC32]-pNPY showed a marked selectivity for the Y5 receptor, while [TOAC2]-pNPY maintained a significant binding also to the Y2 receptor subtype. The modifications of the native peptide structure caused by the introduction of TOAC were examined by circular dichroism. In order to determine the rotational correlation time of the spin probes, electron paramagnetic resonance measurements were performed in solution and in the presence of liposomes. This allowed us to evaluate the backbone dynamics of the different parts of the NPY molecule in the free and membrane bound states. The results of these studies showed that NPY Interacts with liposomes by using the C-terminal alpha-helix while the N-terminal tail retains a flexibility that is comparable to that of the peptide in solution as already shown by NMR studies on DPC micelles. Furthermore, we demonstrated that TOAC-labelllng is a valuable tool to investigate changes in the backbone conformation and dynamics. This may be of major importance for peptides and small proteins when they bind to cell membranes.

Fluorescence-labelled analogs of NPY, a 36-amino acid peptide amide, were synthesized by solid-phase peptide synthesis and used for fluorescence-resonance energy transfer studies to investigate the conformation. Energy-transfer efficiency measurements in different media at the concentration of 10 M are in agreement with a model of the NPY structure proposed by NMR studies (performed at millimolar concentration) in which the C-terminal part of the molecule adopts an ␣-helical conformation while the N-terminal part is flexible. According to this model, the ␣-helix is stabilized by intermolecular hydrophobic interactions because of the formation of dimers. The decrease of the peptide concentration causes a shift of the dimerization equilibrium toward the monomeric form. Energy-transfer efficiency measurements performed at lower concentrations do not support the hypothesis of the folding back of the N-terminal tail onto the C-terminal ␣-helix to yield the so-called "PP-fold" conformation. This structure is observed in the crystal structure of avian pancreatic polypeptide, a member of the NPY peptide hormone family, and it has been considered to be the bioactive one. Our results complete the structural characterization of NPY in solution at concentration ranges in which NMR experiments are not feasible. Furthermore, these results open the way to study the conformation of the receptor-bound ligand.