title: Metal ion-dependent, reversible, protein filament formation by designed beta-roll polypeptides.
abstract: BACKGROUND
  A right-handed, calcium-dependent beta-roll structure found in secreted proteases and repeat-in-toxin proteins was used as a template for the design of minimal, soluble, monomeric polypeptides that would fold in the presence of Ca2+. Two polypeptides were synthesised to contain two and four metal-binding sites, respectively, and exploit stacked tryptophan pairs to stabilise the fold and report on the conformational state of the polypeptide.
  
  
  RESULTS
  Initial analysis of the two polypeptides in the presence of calcium suggested the polypeptides were disordered. The addition of lanthanum to these peptides caused aggregation. Upon further study by right angle light scattering and electron microscopy, the aggregates were identified as ordered protein filaments that required lanthanum to polymerize. These filaments could be disassembled by the addition of a chelating agent. A simple head-to-tail model is proposed for filament formation that explains the metal ion-dependency. The model is supported by the capping of one of the polypeptides with biotin, which disrupts filament formation and provides the ability to control the average length of the filaments.
  
  
  CONCLUSIONS
  Metal ion-dependent, reversible protein filament formation is demonstrated for two designed polypeptides. The polypeptides form filaments that are approximately 3 nm in diameter and several hundred nm in length. They are not amyloid-like in nature as demonstrated by their behaviour in the presence of congo red and thioflavin T. A capping strategy allows for the control of filament length and for potential applications including the "decoration" of a protein filament with various functional moieties.
authors.affiliation: Protein Engineering Network Centres of Excellence, 750 Heritage Medical Research Centre, Edmonton, AB, T6G 2S2, Canada. scottera@post.queensu.ca
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Scotter
authors.forename: Meng
authors.initials: M
authors.lastname: Guo
authors.forename: Melanie M
authors.initials: MM
authors.lastname: Tomczak
authors.forename: Margaret E
authors.initials: ME
authors.lastname: Daley
authors.forename: Robert L
authors.initials: RL
authors.lastname: Campbell
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Oko
authors.forename: David A
authors.initials: DA
authors.lastname: Bateman
authors.forename: Avijit
authors.initials: A
authors.lastname: Chakrabartty
authors.forename: Brian D
authors.initials: BD
authors.lastname: Sykes
authors.forename: Peter L
authors.initials: PL
authors.lastname: Davies
mesh_terms: D000595:Amino Acid Sequence; D000682:Amyloid / Q000737:chemistry; D002942:Circular Dichroism; D007811:Lanthanum / Q000494:pharmacology*; D008027:Light; D008854:Microscopy, Electron; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D010455:Peptides / Q000737:chemistry*; D011108:Polymers / Q000737:chemistry; D020836:Protein Structure, Quaternary / Q000187:drug effects; D017433:Protein Structure, Secondary / Q000187:drug effects; D017434:Protein Structure, Tertiary; D011550:Pseudomonas aeruginosa / Q000201:enzymology; D012542:Scattering, Radiation; D012697:Serine Endopeptidases / Q000737:chemistry / Q000648:ultrastructure
chemical_list: D000682:Amyloid; D010455:Peptides; D011108:Polymers; D007811:Lanthanum; D012697:Serine Endopeptidases