CB1 cannabinoid receptor-phosphorylated fourth intracellular loop structure-function relationships.

A peptide comprising the juxtamembrane C-terminal intracellular loop 4 (IL4) of the CB1 cannabinoid receptor possesses three Serine residues (Ser402, Ser411 and Ser415). Here we report the effect of Ser phosphorylation on the CB1 IL4 peptide conformation and cellular signaling functions using nuclear magnetic resonance spectroscopy, circular dichroism, G protein activation and cAMP production. Circular dichroism studies indicated that phosphorylation at various Ser residues induced helical structure in different environments. NMR data indicates that helical content varies in the order of IL4pSer411 > IL4pSer415 > IL4 > IL4pSer402. The efficacy of phosphorylated IL4 peptides in activating Go and Gi3 ([35S]GTPγS binding) and inhibiting cAMP accumulation in N18TG2 cells were correlated with helicity changes. Treatment of cells with bradykinin, which activates PKC, augmented CB1-mediated inhibition of cAMP accumulation, and this was reversed by a PKC inhibitor, suggesting that phosphorylation of serine might be a physiologically relevant modification in vivo. We conclude that phosphorylation-dependent alterations of helicity of CB1 IL4 peptides can increase efficacy of G protein signaling.

Molecular modeling of neurokinin B and tachykinin NK3 receptor complex.

The tachykinin receptor NK3 is a member of the rhodopsin family of G-protein coupled receptors. The NK3 receptor has been regarded as an important drug target due to diverse physiological functions and its possible role in the pathophysiology of psychiatric disorders, including schizophrenia. The NK3 receptor is primarily activated by the tachykinin peptide hormone neurokinin B (NKB) which is the most potent natural agonist for the NK3 receptor. NKB has been reported to play a vital role in the normal human reproduction pathway and in potentially life threatening diseases such as pre-eclampsia and as a neuroprotective agent in the case of neurodegenerative diseases. Agonist binding to the receptor is a critical event in initiating signaling, and therefore a characterization of the structural features of the agonists can reveal the molecular basis of receptor activation and help in rational design of novel therapeutics. In this study a molecular model for the interaction of the primary ligand NKB with its G-protein coupled receptor NK3 has been developed. A three-dimensional model for the NK3 receptor has been generated by homology modeling using rhodopsin as a template. A knowledge based docking of the NMR derived bioactive conformation of NKB to the receptor has been performed utilizing limited ligand binding data obtained from photoaffinity labeling and site-directed mutagenesis studies. A molecular model for the NKB-NK3 receptor complex obtained sheds light on the topographical features of the binding pocket of the receptor and provides insight into the biochemical data currently available for the receptor.

Three-dimensional structure of Phyllomedusin, a NK1 receptor agonist bound to dodecylphosphocholine micelles.

Phyllomedusin, an amphibian tachykinin decapeptide, has been shown to be selective for Neurokinin 1 receptor. Because the micelle-associated structure may be relevant to the Phyllomedusin-receptor interaction, the three-dimensional structure of the Phyllomedusin in aqueous and micellar environments has been studied by two-dimensional proton nuclear magnetic resonance (2D (1)H NMR spectroscopy) and distance geometry calculations. Sequence specific resonance assignments of protons have been made from correlation spectroscopy (TOCSY, DQF-COSY) and NOESY spectroscopy. The interproton distance constraints and dihedral angle constraints have been utilized to generate a family of structures using DYANA. The CD and NMR results show that, while in water Phyllomedusin prefers to be in an extended chain conformation, whereas in the presence of dodecylphosphocholine micelles, a membrane model system, a partial helical conformation is induced. Analysis of NMR data indicates that the global fold of Phyllomedusin can be explained in terms of equilibrium between 3(10)-helix and alpha-helix from residue 4 to 10. An extended highly flexible N-terminus displays some degree of order and a possible turn structure. A comparison between the conformational features of Phyllomedusin and different Neurokinin 1 receptor agonist indicates several common features in the distribution of hydrophobic and hydrophilic residues. The conformational similarities suggest that the molecules interact with receptor in an analogous manner.