Visualization of highly ordered striated domains induced by transmembrane peptides in supported phosphatidylcholine bilayers

We used atomic force microscopy (AFM) to study the lateral organization of transmembrane TmAW(2)(LA)(n)W(2)Etn peptides (WALP peptides) incorporated i n phospholipid bilayers. These well-studied model peptides consist of a hyd rophobic alanine-leucine stretch of variable length, flanked on each side b y two tryptophans. They were incorporated in saturated phosphatidylcholine (PC) vesicles, which were deposited on a solid substrate via the vesicle fu sion method, yielding hydrated gel-state supported bilayers. At low concent rations (1 mol %) WALP peptides induced primarily line-type depressions in the bilayer. In addition, striated lateral domains were observed, which inc reased in amount and size (from 25 nm up to 10 mu m) upon increasing peptid e concentration. At high peptide concentration (10 mol %), the bilayer cons isted mainly of striated domains. The striated domains consist of line-type depressions and elevations with a repeat distance of 8 nm, which form an e xtremely ordered, predominantly hexagonal pattern. Overall, this pattern wa s independent of the length of the peptides (19-27 amino acids) and the len gth of the lipid acyl chains (16-18 carbon atoms). The striated domains cou ld be pushed down reversibly by the AFM rip and are thermodynamically stabl e. This is the first direct visualization of ct-helical transmembrane pepti de-lipid domains in a bilayer. We propose that these striated domains consi st of arrays of WALP peptides and fluidlike PC molecules, which appear as l ow lines. The presence of the peptides perturbs the bilayer organization, r esulting in a decrease in the tilt of the lipids between the peptide arrays . These lipids therefore appear as high lines.