STRUCTURAL BASIS FOR THE BINDING OF A GLOBULAR ANTIFREEZE PROTEIN TO ICE

ANTIFREEZE proteins (AFPs) have the unique ability to adsorb to ice an d inhibit its growth(1). Many organisms ranging from fish to bacteria use AFPs to retard freezing or lessen the damage incurred upon freezin g and thawing(2-6). The ice-binding mechanism of the long linear alpha -helical type I AFPs has been attributed to their regularly spaced pol ar residues matching the ice lattice along a pyramidal plane(7,8). In contrast, it is not known how globular antifreeze proteins such as typ e III AFP that lack repeating ice-binding residues bind to ice. Here w e report the 1.25 Angstrom crystal structure of recombinant type III A FP (QAE isoform(9)) from eel pout (Macrozoarces americanus), which rev eals a remarkably hat amphipathic ice-binding site where five hydrogen -bonding atoms match two ranks of oxygens on the {10 (1) over bar 0} i ce prism plane in the [0001] direction, giving high ice binding affini ty 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.