Effect of sequence hydrophobicity and bilayer width upon the minimum length required for the formation of transmembrane helices in membranes.

The minimum hydrophobic length necessary to form a transmembrane (TM) helix in membranes was investigated using model membrane-inserted hydrophobic helices. The fluorescence of a Trp at the center of the sequence and its sensitivity to quenching were used to ascertain helix position within the membrane. Peptides with hydrophobic cores composed of poly(Leu) were compared to sequences containing a poly 1:1 Leu:Ala core (which have a hydrophobicity typical of natural TM helices). Studies varying bilayer width revealed that the poly(Leu) core peptides predominately formed a TM state when the bilayer width exceeded hydrophobic sequence length by (i.e. when negative mismatch was) up to approximately 11-12 A (e.g. the case of a 11-12 residue hydrophobic sequence in bilayers with a biologically relevant width, i.e. dioleoylphosphatidylcholine (DOPC) bilayers), while poly(LeuAla) core peptides formed predominantly TM state with negative mismatch of up to 9 A (a 13 residue hydrophobic sequence in DOPC bilayers). This indicates that minimum length necessary to form a predominating amount of a TM state (minimum TM length) is only modestly hydrophobicity-dependent for the sequences studied here, and a formula that defines the minimum TM length as a function of hydrophobicity for moderately-to-highly hydrophobic sequences was derived. The minimum length able to form a stable TM helix for alternating LeuAla sequences, and that for sequences with a Leu block followed by an Ala block, was similar, suggesting that a hydrophobicity gradient along the sequence may not be an important factor in TM stability. TM stability was also similar for sequences flanked by different charged ionizable residues (Lys, His, Asp). However, ionizable flanking residues destabilized the TM configuration much more when charged than when uncharged. The ability of short hydrophobic sequences to form TM helices in membranes in the presence of substantial negative mismatch implies that lipid bilayers have a considerable ability to adjust to negative mismatch, and that short TM helices may be more common than generally believed. Factors that modulate the ability of bilayers to adjust to mismatch may strongly affect the configuration of short hydrophobic helices.