Structural basis for the binding of a globular antifreeze protein to ice

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Abstract
ANTIFREEZE proteins (AFPs) have the unique ability to adsorb to ice and inhibit its growth1. Many organisms ranging from fish to bacteria use AFPs to retard freezing or lessen the damage incurred upon freezing and thawing2–6. The ice-binding mechanism of the long linear α-helical type I AFPs has been attributed to their regularly spaced polar residues matching the ice lattice along a pyramidal plane7,8. In contrast, it is not known how globular antifreeze proteins such as type III AFP that lack repeating ice-binding residues bind to ice. Here we report the 1.25 Å crystal structure of recombinant type III AFP (QAE iso-form9) from eel pout (Macrozoarces americanus), which reveals a remarkably flat amphipathic ice-binding site where five hydrogen-bonding atoms match two ranks of oxygens on the {10¯0} ice prism plane in the (0001) direction, giving high ice-binding affinity and specificity. This binding site, substantiated by the structures and properties of several ice-binding site mutants, suggests that the AFP occupies a niche in the ice surface in which it covers the basal plane while binding to the prism face.