Mixing antifreeze protein types changes ice crystal morphology without affecting antifreeze activity.

All three fish antifreeze protein types (I, II and III) inhibit the growth of ice to form hexagonal bipyramidal ice crystals of characteristic morphology. Mixtures of these different antifreezes produced ice crystals of hybrid shapes and dimensions, consistent with the different antifreeze types binding to the same ice surfaces. The activity of the mixtures was independent of the proportions of the iso-active antifreeze protein stocks present, indicating that the different antifreezes neither attenuated nor potentiated each other's activity. We suggest that antifreeze protein molecules are independently active and do not require protein-protein interactions for ice-binding.