Crystal Structure of β-Helical Antifreeze Protein Points to a General Ice Binding Model

Abstract Reported here is the 2.3 A resolution crystal structure of spruce budworm ( Choristoneura fumiferana ) antifreeze protein (CfAFP), solved by single anomalous scattering. The structure reveals an extremely regular left-handed β-helical platform consisting of 15-amino acid loops with a repetitive Thr-X-Thr motif displayed on one of the helix's three faces. This motif results in a two-dimensional array of threonine residues in an identical orientation to those in the nonhomologous, right-handed β-helical beetle AFP from Tenebrio molitor (TmAFP). The CfAFP structure led us to reevaluate our ice binding model, and the analysis of three possible modes of docking gives rise to a binding mechanism based on surface complementarity. This general mechanism is applicable to both fish and insect AFPs.