The presence of pS₇₀₂ and pY₆₈₆ within a segment of the PECAM-1 cytoplasmic domain spanning residues 684-711 affects its interaction with anionic phospholipid-containing vesicles. (A) CD spectra of PECAM-1_684-711 in the presence of lipid vesicles (6mM) that contained 100% DMPC, decreasing amounts of DMPC and increasing amounts of DMPS, or 100% DMPS. Note that PECAM-1_684-711 exhibited decreased ellipticity at 208 and 222 nm in the presence of vesicles that contained increasing amounts of DMPS relative to vesicles that contained 100% DMPC or buffer alone, which is consistent with increased α-helical content in the presence of anionic phospholipids. (B) CD spectra of PECAM-1_684-711 peptides that contained no phosphorylated residues (684-711) phosphoserine at position 702 (684-711 pS₇₀₂), or both phosphoserine at position 702 and phosphotyrosine at position 686 (684-711 pY₆₈₆ pS₇₀₂) in the presence of vesicles (6mM) that contained 100% DMPS. (C) CD ellipticity measurements at 222 nm of unphosphorylated, pS₇₀₂-containing, or pY₆₈₆- and pS₇₀₂-containing PECAM-1_684-711 peptides in the presence of increasing concentrations of vesicles (0-6mM) composed of 100% DMPS. Note that the magnitude of the drop in ellipticity at 222 nm was most dramatic with unphosphorylated peptide (slope −0.48), reduced with the peptide that contained pS₇₀₂ (slope −0.23), and almost absent with the peptide that contained both pY₆₈₆ and pS₇₀₂ (slope −0.16). Linear regression analysis revealed that differences between the slopes of the lines were very significant (P < .01). These results indicate that the phosphorylated peptides were increasingly insensitive to DMPS-induced α-helix formation.