Mapping of Disulfide Bridges in Antifreeze Proteins from Overwintering Larvae of the Beetle Dendroides canadensis

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Abstract
Antifreeze proteins (AFPs) have been identified in certain high-latitude marine fish, insects and other terrestrial arthropods, and plants. Despite considerable structural variation, the mechanisms of their noncolligative antifreeze activity are probably quite similar. AFPs hydrogen bond onto the surface of potential seed ice crystals at preferred growth sites, thereby preventing growth of the crystals. AFPs from overwintering larvae of the beetle Dendroides canadensis are among the most active AFPs. These 8.7-kDa proteins consist of seven 12- or 13-mer repeating units. Their most striking feature is the location of cysteines every six residues throughout their length. Consequently, identification of the disulfide linkages of these cysteines is essential to understanding the structure of these AFPs. This study demonstrated that all of the 16 Cys residues in the Dendroides AFPs are disulfide bridged. All of the seven 12- or 13-mer repeats have internal disulfide bridges, and in all but the first repeat the Cys residues at positions 1 and 7 of the repeats are linked. In repeat 1 the Cys at position 1 is linked to the Cys at position 10, rather than the Cys at position 7 as in the other repeats, and the Cys at position 7 of the first repeat is linked to a Cys at position 4 of the second repeat. The disulfide bridges probably function to position the hydrophilic side chains of serine and threonine residues so that they hydrogen bond with ice.