ALAMETHICIN PYROMELLITATE - AN ION-ACTIVATED CHANNEL-FORMING PEPTIDE

The synthesis and characterization of alamethicin pyromellitate (Alm-P M), a derivative of the channel-forming peptide alamethicin bearing th ree negative charges at the C-terminus, is described. The self-associa tion of Alm-PM in small unilamellar vesicles of dioleoylphosphatidylch oline (DOPC), monitored using circular dichroism (CD) spectroscopy, oc curs much less readily than the self-association of unmodified alameth icin. Channel formation by Alm-PM also occurs less readily and exhibit s a higher voltage threshold for activation in planar lipid bilayers a nd in lipid vesicles. An increase in the salt concentration, and parti cularly the addition of calcium ions, promotes Alm-PM self-association as monitored by CD spectroscopy. Calcium also facilitates channel for mation by Alm-PM both in planar lipid bilayers and in lipid vesicles b y lowering the voltage threshold for activation. Thus Alm-PM behaves a s an ion-activated ion channel. These results indicate that the self-a ssociation of alamethicin-like peptides in membranes is critical for c hannel formation and that transmembrane flip-flop of peptide helices i s not required. In addition, these results demonstrate that the activi ty of channel-forming peptides may be controlled by controlling the pr ocess of self-association.