The ion-channel activity of longibrachins LGA I and LGB II: effects of Pro-2/Ala and Gln-18/Glu substitutions on the alamethicin voltage-gated membrane channels

Longibrachins LGA I (Ac Aib Ala Aib Ala Aib(5) Ala Gin Aib Val Aib(10) Gly Leu Aib Pro Val(15) Aib Aib Gin Gin Pheol(20) with Aib: alpha-aminoisobutyr ic acid, pheol: phenylalaninol) and LGB II are two homologous 20-residue lo ng-sequence peptaibols isolated from the fungus Trichoderma longibrachiatum that differ between them by a Gln-18/Glu substitution. They distinguish fr om alamethicin by a Pro-2 for Ala replacement, which allowed to examine for the first time with natural Aib-containing analogues, the effect of Pro-2 on the ion-channel properties exhibited by alamethicin. The influence of th ese structural modifications on the voltage-gated ion-channel forming activ ity of the peptides in planar lipid bilayers were analysed. The general 'ba rrel-stave' model of ion-channel activity, already described for alamethici n, was preserved with both longibrachins. The negatively charged LGB II pro moted higher oligomerisation levels, which could presumably dilute the repu lsive effect of the negative Glu ring near the entrance of the channel and resulted in lower lifetimes of the substates, confirming the strong anchor of the peptide C-terminus at the cis-interface. Reduction of the channel li fetimes was observed for the longibrachins, compared to alamethicin. This a rgues for a better stabilisation of the channels formed by peptaibols havin g a proline at position 2, which results in better anchoring of the peptide monomer N-terminus at the trans-bilayer interface. Qualitative assays of t he temperature dependence on the neutral longibrachin channel properties de monstrated a high increase of channel lifetimes and a markedly reduced volt age-sensitivity when the temperature was decreased, showing that such condi tions may allow to study the channel-forming properties of peptides leading to fast current fluctuations. (C) 1999 Elsevier Science B.V. All rights re served.