Ice-binding structure and mechanism of an antifreeze protein from winter flounder

ANTIFREEZE proteins provide fish with protection against the freezing effect of polar environments by binding to ice surfaces and inhibiting growth of ice crystals. We present the X-ray crystal structure at 1.5 Å resolution of a lone α-helical antifreeze protein from winter flounder, which provides a detailed look at its icebinding features. These consist of four repeated ice-binding motifs, the side chains of which are inherently rigid or restrained by pair-wise side-chain interactions to form a flat binding surface. Elaborate amino- and carboxy-terminal cap structures are also present, which explain the protein's rich α-helical content in solution. We propose an ice-binding model that accounts for the binding specificity of the antifreeze protein along the <011¯2> axes of the {202¯1} ice planes1.