Model membranes composed of various lipid mixtures can segregate into liquid-ordered (L-o) and liquid-disordered (L-d) phases. In this study, lipid vesicles composed of mainly L-o or L-d phases as well as complex lipid systems representing the cytosolic leaflet of the myelin membrane were characterized by fluorescence resonance energy transfer with a donor/acceptor pair that preferentially partitioned into L-o or L-d phases, respectively. The fluidity of the lipid systems containing >30% cholesterol was modulated in the presence of the amphipathic peptide melittin. With all the studied lipid systems, melittin attained an alpha-helical conformation as determined by CD spectroscopy and attained varying degrees of membrane association and penetration as determined by intrinsic Trp fluorescence. The other protein domain utilized was a putative amphipathic helical peptide derived from the cytosolic C-terminal sequence of proteolipid protein (PLP) which is the most abundant protein in the myelin membrane. The C-terminal PLP peptide transitioned from a random coil to an alpha-helix in the presence of trifluoroethanol. Upon interacting with each of lipid vesicle system, the PLP peptide also folded into a helix; however, at high concentrations of the peptide with fluid lipid systems, associated helices transmuted into a beta-sheet conformer. The membrane-associated aggregation of the cytosolic C-termini could be a mechanism by which the transmembrane PLP multimerizes in the myelin membrane.