No abstract

The third helix of antennapedia homeodomain pAntp-(43 -58) can translocate through cell membrane and has been used as an intracellular vehicle for delivering peptides and oligonucleotides. The conformational and associative behaviour of two peptidic vectors pAntp-(43 -58) and [Pro50]pAntp-(43 -58) has been analyzed by different biophysical methods. pAntp-(43 -58) adopts an amphipathic helical structure in 30 % (by vol.) hexafluoroisopropanol, in perfluoro-tert-butanol and in the presence of SDS micelles.CD spectra indicate that the conformation of [Pro50]pAntp-(43 -58) in contrast to pAntp-(43 -58) is independent of the media used. 'H-NMR spectroscopy in SDS micelles or in perfluoro-tert-butanol allows detection of aggregated peptides probably in a ribbon 2, type conformation. These conformations became the predominant structure when Gln50 was replaced by ProSO. Interproton-distance restraints derived from NOE measurements have been classified in two groups corresponding to two types of structures: a-helix and essentially extended structures. Consecutive CHa(i)lCHa(i+ 1) NOES are only compatible with aggregates. Simulated annealing calculation of dimeric structure agrees with 4 and y/ angles in the p-sheet and y-turn regions. Fluorescence spectroscopy analysis has shown that the indole groups of both peptides penetrate into SDS micelles; both peptides also induce the formation of micelles at very low concentration of SDS (20 pM). Similar interaction was observed with reverse-phase micelles made of bis(2-ethyhexyl) sodium sulfosuccinate and small unilamellar vesicles (SUV) made of a mixture of phosphatidylcholine/phosphatidylserine. 3'P-NMR of vesicles (SUV and large unilamellar vesicles) indicated that the addition of pAntp analogues did not affect the size of phosphatidylcholine/phosphatidylserine vesicles. The addition of pAntp analogues to lipidic dispersions modulates lipid polymorphism in different ways depending on the mixtures of acidic lipids.

Constrained analogues of phenylalanine have been conceptually designed for analyzing the binding pockets of Phe7 (S7) and Phe8 (S8), two aromatic residues important for the pharmacological properties of SP, i.e., L-tetrahydroisoquinoleic acid, L-diphenylalanine, L-9-fluorenylglycine (Flg), 2-indanylglycine, the diastereomers of L-1-indanylglycine (Ing) and L-1-benz[f]indanylglycine (Bfi), and the Z and E isomers of dehydrophenylalanine (delta ZPhe, delta EPhe). Binding studies were performed with appropriate ligands and tissue preparations allowing the discrimination of the three tachykinin binding sites, NK-1, NK-2, and NK-3. The potencies of these agonists were evaluated in the guinea pig ileum bioassay. According to the binding data, we can conclude that the S7 subsite is small, only the gauche (-) probe [(2S,3S)-Ing7]SP presents a high affinity for specific NK-1 binding sites. Surprisingly, the [delta EPhe7]SP analogue, which projects the aromatic ring toward the trans orientation, is over 40-fold more potent than the Z isomer, [delta ZPhe7]SP. A plausible explanation of these conflictual results is that either the binding protein quenches the minor trans rotamer of [(2S,3S)-Ing7]SP in solution or this constrained amino acid side chain rotates when inserted in the protein. In position 8, the high binding affinities of [Flg8]SP and [(2S,3S)-Bfi8]SP suggest that the S8 subsite is large enough to accept two aromatic rings in the gauche (-) and one aromatic ring in the trans direction. Peptides bearing two conformational probes in positions 7, 8, or 9 led to postulate that S7, S8, and S9 subsites are independent from each other. The volumes available for side chains 7 and 8 can be estimated to be close to 110 and 240 A3, respectively. The large volume of the S8 subsite raises question on the localization of the SP-binding site in the NK-1 receptor. If SP were to bind in the transmembrane domains, the cleft defined by the seven transmembrane segments must rearrange during the binding process in order to bind a peptide in an alpha-helical structure and at least one large binding subsite in position 8. Thus, indirect topographical analysis with constrained amino acids might contribute to the analysis of the receptor/ligand dynamics. Finally, this study demonstrates that a good knowledge of the peptidic backbone structure and a combination of constrained amino acids are prerequisites to confidently attribute the preferred orientation(s) of an amino acid side chain.