Alamethicin Pyromellitate: An Ion-Activated Channel-Forming Peptide
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The synthesis and characterization of alamethicin pyromellitate (Alm-PM), a derivative of the channel-forming peptide alamethicin bearing three negative charges at the C-terminus, is described. The self-association of Alm-PM in small unilamellar vesicles of dioleoylphosphatidylcholine (DOPC), monitored using circular dichroism (CD) spectroscopy, occurs much less readily than the self-association of unmodified alamethicin. Channel formation by Alm-PM also occurs less readily and exhibits a higher voltage threshold for activation in planar lipid bilayers and in lipid vesicles. An increase in the salt concentration, and particularly the addition of calcium ions, promotes Alm-PM self-association as monitored by CD spectroscopy. Calcium also facilitates channel formation by Alm-PM both in planar lipid bilayers and in lipid vesicles by lowering the voltage threshold for activation. Thus Alm-PM behaves as an ion-activated ion channel. These results indicate that the self-association of alamethicin-like peptides in membranes is critical for channel formation and that transmembrane flip-flop of peptide helices is not required. In addition, these results demonstrate that the activity of channel-forming peptides may be controlled by controlling the process of self-association.