Sensitivity of single membrane-spanning alpha-helical peptides to hydrophobic mismatch with a lipid bilayer: Effects on backbone structure, orientation, and extent of membrane incorporation

The extent of matching of membrane hydrophobic thickness with the hydrophob ic length of transmembrane protein segments potentially constitutes a major director of membrane organization. Therefore, the extent of mismatch that can be compensated, and the types of membrane rearrangements that result, c an provide valuable insight into membrane functionality. In the present stu dy, a large family of synthetic peptides and Lipids is used to investigate a range of mismatch situations. Peptide conformation, orientation, and exte nt of incorporation are assessed by infrared spectroscopy, tryptophan fluor escence, circular dichroism, and sucrose gradient centrifugation. It is sho wn that peptide backbone structure is not significantly affected by mismatc h, even when the extent of mismatch is large. Instead, this study demonstra tes that for tryptophan-flanked peptides the dominant response of a membran e to large mismatch is that the extent of incorporation is reduced, when th e peptide is both too short and too long. With increasing mismatch, a small er fraction of peptide is incorporated into the lipid bilayer, and a larger fraction is present in extramembranous aggregates. Relatively long peptide s that remain incorporated in the bilayer have a small tilt angle with resp ect to the membrane normal. The observed effects depend on the nature of th e flanking residues: long tryptophan-flanked peptides do not associate well with thin bilayers, while equisized lysine-flanked peptides associate comp letely, thus supporting the notion that tryptophan and lysine interact diff erently with membrane-water interfaces. The different properties that aroma tic and charged flanking residues impart on transmembrane protein segments are discussed in relation to protein incorporation in biological systems.