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

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 thawing^2–6. 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 plane^7,8. 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.