Fibril formation by an amphipathic alpha-helix-forming polypeptide produced by gene engineering

The alpha(3)-peptide, which comprises three repeats of the seven-residue sequence Leu-Glu-Thr-Leu-Ala-Lys-Ala, was designed to form an amphipathic alpha-helix with a hydrophobic surface by Leu residues and a hydrophilic surface by Glu and Lys residues, thus yielding a coiled coil or a helical bundle structure through their association. The alpha(3)-peptide was produced by gene engineering using Escherichia coli. Association to a tetramer had been demonstrated by sedimentation equilibrium analysis in a previous study. In addition to tetramerization, electron microscopic observation revealed that the alpha(3)-peptide formed ''fibrils'' 5 to 10 nm in width in 10 mM potassium phosphate/0.1 M KCl (pH 6.0). By increasing the salt concentration, the alpha(3)-peptide formed much larger ''fibers'' assembled from many ''filaments'' running along the long axis, each of which was thinner and longer than those observed at lower salt concentration. Hydrophobic interaction is considered to be a main force responsible for forming the fibrous structure. However, the electrostatic features of the alpha(3)-peptide seem to affect fibril assembly, since the shape and size of the fibrous structure were altered by the ionic strength of the solution. To our knowledge, this is the first report to describe formation of a fibrous structure by a de novo designed alpha-helix-forming peptide, and thus the alpha(3)-peptide with its simple sequence is considered to be a potential model peptide for investigating the molecular mechanisms of fibril formation by peptides or proteins.