title: Helix-turn-helix peptides that form alpha-helical fibrils: turn sequences drive fibril structure.
abstract: Models of apolipoprotein A-I (apo A-I), the main protein of high-density lipoprotein, predict that it contains 10 amphiphilic, alpha-helical segments connected by turns. We synthesized four peptides with two identical 18-residue, amphiphilic, alpha-helical segments (Anantharamaiah, G. M., et al. (1985) J. Biol. Chem. 260, 10248-10255) connected by putative turn sequences from apo A-I: (1) Ac-DWLKAFYDKVAEKLKEAFKVEPLRADWLKAFYDKVAEKLKEAF-NH2, (2) Ac-DWLKAFYDKVAEKLKEAFGLLPVLEDWLKAFYDKVAEKLKEAF-NH2, (3) Ac-DWLKAFYDKVAEKLKEAFKVQPYLDDWLKAFYDKVAEKLKEAF-NH2, and (4) Ac-DWLKAFYDKVAEKLKEAFNGGARLADWLKAFYDKVAEKLKEAF-NH2. Surprisingly, peptides 1-3 formed fibrils after incubation (37 degrees C, 10 mM sodium phosphate, pH 7.60), but in contrast to beta-sheet amyloid fibrils, these did not bind thioflavin T and they induced a blue shift in the spectrum of Congo red. CD (peptides 1-3) and FTIR (peptides 1 and 2) of the fibrils showed significant alpha-helical character. Synchrotron X-ray fiber diffraction on a magnetically aligned sample of 1 confirmed the alpha-helical character in the fibrils and indicated that the helical axes are oriented perpendicular to the fibril axis. In contrast, peptide 4, containing two Gly residues but no Pro in the turn, formed only a small amount of nonfibrillar precipitate after prolonged incubation. Peptide 4P (peptide 4 with a Pro in place of the central Ala) and peptide 5, containing a PEG block in lieu of the central turn, were similar to peptide 4 in not forming fibrils, possibly because the region linking the helices was unstructured. These studies indicate that varying turn sequences between longer amphiphilic alpha-helical segments can drive the structure of fibrils.
authors.affiliation: Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Kristi L
authors.initials: KL
authors.lastname: Lazar
authors.forename: Hélène
authors.initials: H
authors.lastname: Miller-Auer
authors.forename: Godfrey S
authors.initials: GS
authors.lastname: Getz
authors.forename: Joseph P R O
authors.initials: JP
authors.lastname: Orgel
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Meredith
mesh_terms: D000595:Amino Acid Sequence; D000682:Amyloid / Q000737:chemistry* / Q000648:ultrastructure*; D016632:Apolipoprotein A-I / Q000737:chemistry* / Q000648:ultrastructure*; D052160:Benzothiazoles; D002942:Circular Dichroism; D003224:Congo Red / Q000737:chemistry; D005456:Fluorescent Dyes / Q000737:chemistry; D008051:Lipid Bilayers / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry; D017433:Protein Structure, Secondary; D017550:Spectroscopy, Fourier Transform Infrared; D013844:Thiazoles / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: D000682:Amyloid; D016632:Apolipoprotein A-I; D052160:Benzothiazoles; D005456:Fluorescent Dyes; D008051:Lipid Bilayers; D010455:Peptides; D013844:Thiazoles; C009462:thioflavin T; D003224:Congo Red