On the differential impact of point mutations of antifreeze protein type III on ice recrystallization inhibition and thermal hysteresis activities

Ice-binding proteins (IBPs) enable cold-adapted organisms to survive by modulating ice growth. IBPs display distinct types of activities, notably thermal hysteresis (TH) activity and ice recrystallization (IRI) activity, but it remains unclear how the different activities relate to IBP structure. Here, we examine how the amino-acid composition of the ice-binding site (IBS) is related to TH and IRI activities, focusing on mutants of a specific IBP type: the moderately active QAE isoform of antifreeze protein type III. We show that point mutations of the IBS of QAE reduce both types of ice-binding activity, but the extent of reduction, and which activity is affected most strongly depends on the mutation type. When a larger wild-type residue is mutated to a smaller residue, we find that TH activity decreases moderately and then reaches a plateau, whereas IRI activity drops more gradually. Conversely, when a smaller wild-type residue is mutated to a larger one, TH activity decreases more significantly. In this latter case, the extent of reduction in both TH and IRI activities appears to depend on the specific mutated residue. These mutation-induced differences in TH and IRI activities suggest that different mutation types can help clarify which IBP features determine TH and IRI activities. Ultimately, these insights may lead to the development of IBPs that are optimized to protect various materials against ice through different strategies: either preventing freezing or tolerating it.