RESUMEN
Synthetic alamethicin analogs, in which all Aib residues had been replaced by Leu (L2) then proline 14 replaced by an alanine (L5), were studied in SDS micelles using circular dichroism and NMR spectroscopy. Nuclear Overhauser effects were used as constraints for molecular modelling. The structures determined for both peptides in SDS micelles were compared with those previously obtained in methanol in order to establish a secondary structure/ionophore activity relationship. Our results indicated that a shortening of peptide helices could be responsible for the observed decrease in ion channel lifetimes. However, the length of helices may not by itself explain the drastic destabilization of channels when Pro14 of alamethicin is replaced by Ala in L5. Indeed analysis of the helical wheel of L5 reveals heterogeneity in the amphipathicity depending on the medium. Thus, loss of amphipathicity seems to underly the observed destabilization of channels.
Asunto(s)
Alameticina/análogos & derivados , Alameticina/química , Modelos Moleculares , Pliegue de Proteína , Estructura Secundaria de Proteína , Secuencia de Aminoácidos , Espectroscopía de Resonancia Magnética , Datos de Secuencia Molecular , Porinas/químicaRESUMEN
Four peptides mimicking the four P-regions of the electric eel sodium channel were chemically synthesized to characterize their secondary structure and their contribution to the channel selectivity. Circular dichroism spectra of these peptides in trifluoroethanol demonstrate an important beta-sheet conformational component. This beta-sheet content is much enhanced upon interaction with phosphatidylcholine small unilamellar vesicles. As expected (and except for P of domain III), no significant voltage-dependence is revealed in either macroscopic or single-channel conductance experiments. The concentrations-dependences of macroscopic conductances suggest that tetramers are the membrane conducting aggregates. In asymmetric ionic conditions, these channels made up of P-peptides were mostly specific for sodium over chloride whilst caesium was largely excluded. Single-channel conductance analysis discloses a moderate selectivity for sodium over potassium for PI and PII. This selectivity is larger with PIII but inverted for PIV. Finally, a control random peptide of the same length and with a comparable mean hydrophibicity was also tested. Its conformation in TFE is mainly unordered and no activity was detected in planar lipid bilayers. The data suggest that the presumed selectivity filter may not assume a circular symmetry and that molecular recognition between the different P-regions has to be taken into account.
Asunto(s)
Conformación Proteica , Canales de Sodio/química , Canales de Sodio/fisiología , Secuencia de Aminoácidos , Animales , Cromatografía Líquida de Alta Presión , Dicroismo Circular , Electrophorus , Activación del Canal Iónico , Membrana Dobles de Lípidos , Potenciales de la Membrana , Datos de Secuencia Molecular , Fragmentos de Péptidos/química , Fosfatidilcolinas , Fosfatidiletanolaminas , Estructura Secundaria de ProteínaRESUMEN
Due to the bend introduced by proline 14 in the conformation of alamethicin (AcUPUAUAQUVUGLUPV UUEQFol), the role of this residue was assumed essential in the barrel-stave model for voltage-gated ion channels. Taking advantage of a previous synthetic alamethicin analogue (L2), in which all eight alpha-aminoisobutyric (U) were replaced by leucines (AcLPLALAQLV LGLLPV LLEQFol), another analogue (L5) was synthesized in order to test the effects of proline-14 substitution by an alanine (AcLPLALAQLVLGLLPVLLEQFol). Previous conductance experiments showed that both high voltage dependence and multistate behavior were conserved. In order to complement these functional results, a conformational study of L5 has been undertaken and compared to L2 using CD, high field nmr, and molecular dynamics. Results show that L5 presents a better ordered structure than L2 particularly in the region of the substitution and in the C-terminal part. These results are discussed as regards the previous hypothesis of the nonessential character of helix bending for the gating of voltage-dependent ion channels.
Asunto(s)
Alameticina/química , Péptidos/química , Prolina , Estructura Secundaria de Proteína , Secuencia de Aminoácidos , Dicroismo Circular , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Datos de Secuencia Molecular , Péptidos/síntesis química , Relación Estructura-ActividadRESUMEN
Alamethicin, a 20-residue peptaibol, induces voltage-dependent ion channels in lipid bilayers according to the barrel-stave model. A synthetic analogue (L2) in which all Aib were replaced by Leu shows a conductance behaviour similar to alamethicin, but channel lifetimes are drastically reduced. Among several hypotheses, a different conformation for L2 might be responsible for this phenomenon by increasing the alpha-helical content (alamethicin presents some 3.0(10)-helical parts) and thus decreasing the length of the transmembrane part. A conformational study of L2 was undertaken using FTIR, CD and NMR spectroscopy, and the secondary structure was compared with alamethicin. These techniques showed an enhanced predominant helical structure as compared to alamethicin. Moreover, the NOE pattern showed an exclusively alpha-helical conformation, resulting in a smaller length of the L2 peptide. This shortening somewhat impedes the complete crossing of the membrane, and could then explain the reduction of its ion-channel lifetimes.