IDENTIFICATION OF THE ICE-BINDING SURFACE ON A TYPE-III ANTIFREEZE PROTEIN WITH A FLATNESS FUNCTION ALGORITHM

Antifreeze proteins (AFPs) adsorb to surfaces of growing ice crystals, thereby arresting their growth. The prevailing hypothesis explains th e nature of adsorption in terms of a match between the hydrophilic sid e chains on the AFP's ice-binding surface (IBS) and the water molecule s on the ice surface. The number and spatial arrangement of hydrogen b onds thus formed have been proposed to account, respectively, for the binding affinity and specificity. The crystal structure of a type III AFP from ocean pout (isoform HPLC-3) has been determined to 2.0-Angstr om resolution. The structure reveals an internal dyad motif formed by two 19-residue. loop-shaped elements. Based on of the flatness observe d on the type I alpha-helical AFP's IBS, an automated algorithm was de veloped to analyze the surface planarity of the globular type III AFP and was used to identify the IBS on this protein. The surface with the highest flatness score is formed by one loop of the dyad motif and is identical to the IBS deduced from earlier mutagenesis studies. Intere stingly, 67% of this surface contains nonpolar solvent-accessible surf ace area. The success of our approach to identifying the IBS on an AFP , without considering the presence of polar side chains, indicates tha t flatness is the first approximation of an IBS. We further propose th at the specificity of interactions between an IBS and a particular ice -crystallographic plane arises from surface complementarity.