title: Flies expand the repertoire of protein structures that bind ice.
abstract: An antifreeze protein (AFP) with no known homologs has been identified in Lake Ontario midges (Chironomidae). The midge AFP is expressed as a family of isoforms at low levels in adults, which emerge from fresh water in spring before the threat of freezing temperatures has passed. The 9.1-kDa major isoform derived from a preproprotein precursor is glycosylated and has a 10-residue tandem repeating sequence xxCxGxYCxG, with regularly spaced cysteines, glycines, and tyrosines comprising one-half its 79 residues. Modeling and molecular dynamics predict a tightly wound left-handed solenoid fold in which the cysteines form a disulfide core to brace each of the eight 10-residue coils. The solenoid is reinforced by intrachain hydrogen bonds, side-chain salt bridges, and a row of seven stacked tyrosines on the hydrophobic side that forms the putative ice-binding site. A disulfide core is also a feature of the similar-sized beetle AFP that is a β-helix with seven 12-residue coils and a comparable circular dichroism spectrum. The midge and beetle AFPs are not homologous and their ice-binding sites are radically different, with the latter comprising two parallel arrays of outward-pointing threonines. However, their structural similarities is an amazing example of convergent evolution in different orders of insects to cope with change to a colder climate and provide confirmation about the physical features needed for a protein to bind ice.
authors.affiliation: Protein Function Discovery Group and Department of Biomedical and Molecular Sciences, Queen's University, Kingston, ON, Canada K7L 3N6.
authors.forename: Koli
authors.initials: K
authors.lastname: Basu
authors.affiliation: Protein Function Discovery Group and Department of Biomedical and Molecular Sciences, Queen's University, Kingston, ON, Canada K7L 3N6.
authors.forename: Laurie A
authors.initials: LA
authors.lastname: Graham
authors.affiliation: Protein Function Discovery Group and Department of Biomedical and Molecular Sciences, Queen's University, Kingston, ON, Canada K7L 3N6.
authors.forename: Robert L
authors.initials: RL
authors.lastname: Campbell
authors.affiliation: Protein Function Discovery Group and Department of Biomedical and Molecular Sciences, Queen's University, Kingston, ON, Canada K7L 3N6 peter.davies@queensu.ca.
authors.forename: Peter L
authors.initials: PL
authors.lastname: Davies
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D021301:Antifreeze Proteins / Q000737:chemistry / Q000378:metabolism*; D004175:Diptera / Q000378:metabolism*; D006031:Glycosylation; D007053:Ice; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid; D053719:Tandem Mass Spectrometry
chemical_list: D021301:Antifreeze Proteins; D007053:Ice
keywords: antifreeze protein; convergent evolution; disulfide-rich; midge; solenoid