Ion channels of N-terminally linked alamethicin dimers: enhancement of cation-selectivity by substitution of Glu for Gln at position 7

Alamethicin forms voltage-gated ion channels that have moderate cation-selectivity. The enhancement of the cation-selectivity by introducing negatively charged residues at positions 7 and 18 has been studied using the tethered homodimers of alamethicin with Q7 and E18 (di-alm-Q7E18) and its analog with E7 and Q18 (di-alm-E7Q18). In the dimeric peptides, monomer peptides are linked at the N-termini by a disulfide bond. Both the peptides formed long lasting ion channels at cis-positive voltages when added to the cis-side membrane. Their long open duration enabled us to obtain current-voltage (I-V(m)) relations and reversal potentials at the single-channel level by applying a voltage ramp during the channel opening. The reversal potentials measured in asymmetric KCl solutions indicated that ionized E7 provided strong cation-selectivity, whereas ionized E18 little influenced the charge selectivity. This was also the case for the macroscopic charge selectivity determined from the reversal potentials obtained by the macroscopic I-V(m) measurements. The results are accounted for by stronger electrostatic interactions between permeant ions and negatively charged residues at the narrowest part of the pore than at the pore mouth.