期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：3
页码：737-742
DOI：10.1073/pnas.1422272112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceA previously unidentified insect antifreeze protein has been characterized in a fly. It is present in the adult stage of midges that emerge from fresh water in the spring and need protection from occasional night frosts. The mature protein has 79 residues and is small and repetitive enough to be reliably modeled as a tightly wound solenoid structure. Each tandem repeat of 10 residues forms a coil of the solenoid that is cross-braced by a disulfide bond. This fold orients a row of seven stacked tyrosine side chains to make a flat surface, which is the predicted ice-binding site of the protein. This protein has no homologs in the database and has, therefore, recently evolved to protect its host from freezing. 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 {beta}-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.
