Protein engineering shows antifreeze activity scales with ice-binding site area

The ice-binding site (IBS) of the 9.6-kDa springtail (Collembola) antifreeze protein from Granisotoma rainieri was identified by mutagenesis. We then studied the proteins activity as a function of IBS area. Its polyproline type II helical bundle fold facilitates changes to both IBS length and width. A one third increase in IBS width, through the addition of a single helix doubled antifreeze activity. A one third decrease in area reduced activity to 10%. A construct engineered with an additional tripeptide turn in each helix displayed a 5-fold decrease in activity. Molecular dynamics suggested that the lengthened IBS is more twisted than the wild type, emphasizing the importance of a flat surface for antifreeze activity.