title: Understanding the mechanism of ice binding by type III antifreeze proteins.
abstract: Type III antifreeze proteins (AFPs) are present in the body fluids of some polar fishes where they inhibit ice growth at subzero temperatures. Previous studies of the structure of type III AFP by NMR and X-ray identified a remarkably flat surface on the protein containing amino acids that were demonstrated to be important for interaction with ice by mutational studies. It was proposed that this protein surface binds onto the (1 0 [\bar 1] 0) plane of ice with the key amino acids interacting directly with the water molecules in the ice crystal. Here, we show that the mechanism of type III AFP interaction with ice crystals is more complex than that proposed previously. We report a high-resolution X-ray structure of type III AFP refined at 1.15 A resolution with individual anisotropic temperature factors. We report the results of ice-etching experiments that show a broad surface coverage, suggesting that type III AFP binds to a set of planes that are parallel with or inclined at a small angle to the crystallographic c-axis of the ice crystal. Our modelling studies, performed with the refined structure, confirm that type III AFP can make energetically favourable interactions with several ice surfaces.
authors.affiliation: Structural Biology Laboratory Department of Chemistry, University of York, Heslington, York Y01 5DD, UK.
authors.forename: A A
authors.initials: AA
authors.lastname: Antson
authors.forename: D J
authors.initials: DJ
authors.lastname: Smith
authors.forename: D I
authors.initials: DI
authors.lastname: Roper
authors.forename: S
authors.initials: S
authors.lastname: Lewis
authors.forename: L S
authors.initials: LS
authors.lastname: Caves
authors.forename: C S
authors.initials: CS
authors.lastname: Verma
authors.forename: S L
authors.initials: SL
authors.lastname: Buckley
authors.forename: P J
authors.initials: PJ
authors.lastname: Lillford
authors.forename: R E
authors.initials: RE
authors.lastname: Hubbard
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D021343:Antifreeze Proteins, Type III / Q000737:chemistry* / Q000378:metabolism*; D001665:Binding Sites; D018360:Crystallography, X-Ray; D005399:Fishes; D005615:Freezing; D007053:Ice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011485:Protein Binding; D017433:Protein Structure, Secondary; D016415:Sequence Alignment; D013696:Temperature; D013816:Thermodynamics
chemical_list: D021343:Antifreeze Proteins, Type III; D007053:Ice