Effect of anionic lipids upon the topography of membrane-inserted hydrophobic helices
To investigate the effect of lipid structure upon the membrane topography of hydrophobic helices, the behavior of hydrophobic peptides was studied in model membrane vesicles. To define topography, fluorescence and fluorescence quenching methods were used to determine the location of a Trp at the center of the hydrophobic sequence. For peptides with cationic residues flanking the hydrophobic sequence, the stability of the transmembrane (TM) configuration (relative to a membrane-bound non-TM state) increased as a function of lipid composition in the order: 1:1 (mol:mol) 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC):1-palmitoyl-2-oleoyl phosphatidylethanolamine (POPE) ∼ 6:4 POPC:cholesterol < POPC ∼ dioleoylphosphatidylcholine (DOPC) < dioleoylphosphatidylglycerol (DOPG) ≤ dioleoylphosphatidylserine (DOPS), indicating that the anionic lipids DOPG and DOPS most strongly stabilized the TM configuration. TM-stabilization was near-maximal at 20-30mol% anionic lipid, physiologically relevant values. TM-stabilization by anionic lipid was observed for hydrophobic sequences with diverse set of sequences (including polyAla), diverse lengths (from 12-22 residues), and various cationic flanking residues (H, R or K), but not when the flanking residues were uncharged. TM-stabilization by anionic lipid was also dependent on the number of cationic residues flanking the hydrophobic sequence, but was still significant with only one cationic residue flanking each end of the peptide. These observations are consistent with TM-stabilizing effects being electrostatic in origin. However, Trp located more deeply in 100mol%DOPS vesicles relative to 100mol%DOPG vesicles, and peptides in DOPS vesicles showed increased helix formation relative to DOPG and all other lipid compositions. These observations fit a model in which DOPS anchors flanking residues near the membrane surface more strongly than does DOPG, and/or increases the stability of the TM state to a greater degree than DOPG. TM-stabilization by anionic lipids was also observed for shifted TM helices (flanked on both sides by cationic residues) in which TM helix transverse movement within the bilayer was suppressed in 20 mol percent anionic lipid containing vesicles compare to the uncharged vesicles.
Topological consequence of hydrophilic mutations and anionic lipid upon the ErbB2 receptor(often over expressed in breast cancer) TM domain was also studied. Mutation at 664 that replaces a V residue with a hydrophilic residue pushed a part of the TM helix out of the membrane toward the N-terminus resulting in a shorter TM helix relative to the longer WT TM helix. Contrary to the artificial shifted TM sequences, 20% anionic lipid did not have any significant TM stabilizing effect upon the mutant ErbB2 shifted TM structures. ErbB2 have an abundance of positively charged residues only at the C-terminal end which can interact with the anionic lipid but the helix shift happened in the opposite direction so the shifted structures was unaffected in the presence of anionic lipids. However, helix shift toward the C-terminus induced by the hydrophilic mutation near the C-terminus region in uncharged vesicles was greatly suppressed in anionic lipid containing vesicles. Hence, TM helix shift can not happen toward the cytoplasmic side of the membrane where the TM protein juxtamembrane (JM) region rich in positively charged residues tend to reside due to their affinity for anionic lipid in cytofacial membrane.