FOLDING OF BETA-SHEET MEMBRANE-PROTEINS - A HYDROPHOBIC HEXAPEPTIDE MODEL

Beta-sheets, in the form of the beta-barrel folding motif, are found i n several constitutive membrane proteins (porins) and in several micro bial toxins that assemble on membranes to form oligomeric transmembran e channels. We report here a first step towards understanding the prin ciples of beta-sheet formation in membranes. In particular, we describ e the properties of a simple hydrophobic hexapeptide, acetyl-Trp-Leu(5 ) (AcWL5), that assembles cooperatively into beta-sheet aggregates upo n partitioning into lipid bilayer membranes from the aqueous phase whe re the peptide is strictly monomeric and random coil. The aggregates, containing 10 to 20 monomers, undergo a relatively sharp and reversibl e thermal unfolding at similar to 60 degrees C. No pores are formed by the aggregates, but they do induce graded leakage of vesicle contents at very high peptide to Lipid ratios. Because beta-sheet structure is not observed when the peptide is dissolved in n-octanol, trifluoroeth anol or sodium dodecyl sulfate micelles, aggregation into beta-sheets appears to be an exclusive property of the peptide in the bilayer memb rane interface. This is an expected consequence of the hypothesis that a reduction in the free energy of partitioning of peptide bonds cause d by hydrogen bonding drives secondary structure formation in membrane interfaces. But, other features of interfacial partitioning, such as side-chain interactions and reduction of dimensionality, must also con tribute. We estimate from our partitioning data that the free energy r eduction per residue for aggregation is about 0.5 kcal mol(-1). Althou gh modest, its aggregate effect on the free energy of assembling beta- sheet proteins can be huge. This surprising finding, that a simple hyd rophobic hexapeptide readily assembles into oligomeric beta-sheets in membranes, reveals the potent ability of membranes to promote secondar y structure in peptides, and shows that the formation of beta-sheets i n membranes is more facile than expected. Furthermore, it provides a b asis for understanding the observation that membranes promote self-ass ociation of beta-amyloid peptides. AcWL5 and related peptides thus pro vide a good starting point for designing peptide models for exploring the principles of beta-sheet formation in membranes. (C) 1998 Academic Press Limited.