title: The pH-dependent assembly of Chaplin E from Streptomyces coelicolor.
abstract: Chaplin E, is one of five self-assembling peptides secreted by Streptomyces coelicolor that assist aerial growth by lowering the surface tension of water. Although the surface activity of a mixture of chaplin peptides has observed to depend on pH, it is unclear how the solvent environment (i.e. pH) influences the structure, assembly and subsequent functionality of these individual peptides. In this study, the conformation and fibril forming propensity of the Chaplin E peptide was assessed as a function of pH using a combination of experimental measurements and molecular dynamics simulations. At an acidic pH of 3.0, Chaplin E retained a random coil structure, whereas at the isoelectric point of 6.7 or a basic pH of 10.0, Chaplin E rapidly formed amyloid fibrils rich in β-sheet structure with high efficiency (>93%). Molecular dynamics simulations indicate the persistence of greater α-helical content at the N-terminus at high pH; this is likely partly due to the lack of electrostatic repulsion between residues His6 and Lys10. Since fibril formation was observed at high but not at low pH, we propose that the presence of an N-terminal α-helix in the monomeric form of Chaplin E is required for aggregation and conversion to β-amyloid fibrils. The pH sensitivity of Chaplin E peptide structure provides a route to control peptide assembly and may be important for the physiological function of this peptide, as a surface active agent in the transition from vegetative to aerial growth and could assist Streptomyces coelicolor in response to environmental fluctuations in pH.
authors.affiliation: The Melbourne School of Engineering and Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Parkville, VIC 3010, Australia.
authors.forename: Mina
authors.initials: M
authors.lastname: Dokouhaki
authors.affiliation: School of Science, RMIT University, VIC 3001, Australia.
authors.forename: Andrew
authors.initials: A
authors.lastname: Hung
authors.affiliation: AgResearch Ltd, Grasslands Research Centre, Tennent Drive, Palmerston North 4442, New Zealand.
authors.forename: Li
authors.initials: L
authors.lastname: Day
authors.affiliation: The Melbourne School of Engineering and Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Parkville, VIC 3010, Australia; The ARC Dairy Innovation Hub, The University of Melbourne, Parkville, Vic 3010, Australia. Electronic address: sgras@unimelb.edu.au.
authors.forename: Sally L
authors.initials: SL
authors.lastname: Gras
mesh_terms: D000682:Amyloid / Q000737:chemistry; D058227:Amyloidogenic Proteins / Q000737:chemistry* / Q000502:physiology; D001426:Bacterial Proteins / Q000737:chemistry / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D017433:Protein Structure, Secondary; D048372:Streptomyces coelicolor / Q000737:chemistry*; D013329:Structure-Activity Relationship; D013501:Surface-Active Agents
chemical_list: D000682:Amyloid; D058227:Amyloidogenic Proteins; D001426:Bacterial Proteins; D013501:Surface-Active Agents
keywords: Circular dichroism; Fibril formation kinetics; Isoelectric focusing gel; Molecular dynamics simulations; Transmission electron microscopy