Interaction of a helical peptide with membrane: Study of alamethicin
Huang, Huey W.
Doctor of Philosophy
Alamethicin is a transmembrane ion channel at low concentration, and a lytic agent of cell membrane at high concentration. It is a small size polypeptide (20 amino residues), and contains a large section of amphiphilic $\alpha$-helix, which is an often-encountered secondary structure motif in membrane active peptides and proteins. Its membrane-active functions, typical secondary structure, and relatively small size made this peptide be an ideal model for studying the interaction of proteins with membranes. This thesis provides two novel methods to obtain structural information of such a peptide-lipid system. By the method of oriented circular dichroism (OCD), we are able to determine the orientation of the alamethicin with respect to the membrane bilayer. Depending on the alamethicin concentration and the water content in the membrane, alamethicin either perpendicularly inserts into the bilayer or binds parallel to the membrane surface. By the method of lamellar x-ray diffraction, we found the membrane bilayer thickness reduced by increasing concentration of alamethicin and decreasing relative humidity. From these two mutually complementary studies we constructed a consistent picture for the interaction between alamethicin and the membrane. When alamethicin concentration is low, the peptide molecules adsorb near the area of lipid head group, which effectively expands the average cross sectional area of the lipid molecules and makes the hydrocarbon chains more disordered, so that the lipid bilayer becomes thinner than pure lipid membrane. When alamethicin concentration reaches the critical point, where membrane structure becomes favorable for the insertion of alamethicin, alamethicin undergoes a transition from the surface state to the insertion state. The insertion of alamethicin would introduce much water into membrane, so the transition would happen only when relative humidity is high. Our alamethicin-lipid interaction model explains the spontaneous insertion of alamethicin at high aqueous concentration causing the lysis of membrane. It also suggests a possible gating mechanism for alamethicin ion channels.
Biophysics; Optics; Biochemistry