Amphipathic beta structure of a leucine-rich repeat peptide

Long tandem arrays of a characteristic leucine-rich repeat motif on the order of 24 amino acids in length have been found in the primary structure of an increasing number of proteins. The most striking feature of these repeats is an amphipathic sequence, with leucine as the predominant hydrophobic residue. Based on this amphipathic sequence and the function of the proteins in which they have been found, the repeats have been proposed to be involved in protein-protein and protein-lipid interactions. As a step toward elucidating the structure and biochemical properties of the leucine-rich repeat motif, we have studied a synthetic leucine-rich repeat peptide (LRP32) representing one of the repeats found in Drosophila chaoptin. We have shown that: (i) LRP32 is soluble in aqueous solution but will bind quantitatively to phospholipid vesicles; (ii) LRP32 has a partial beta structure in aqueous solution and is predominantly a beta structure in the presence of phospholipid; (iii) LRP32 integrates into lipid bilayers to form 60-A intramembrane particles as seen using freeze-fracture electron microscopy (these putative oligomeric structures appear to contain a central aqueous core as indicated by their ability to generate conductances in planar lipid bilayers); and (iv) LRP32-lipid complexes generate 2H NMR spectra characteristic of integral membrane proteins. This study is consistent with LRP32 forming an amphipathic beta sheet. We propose that protein segments containing tandem arrays of leucine-rich repeats also may form amphipathic beta sheets.