Direct evidence for pinning of single, ice-bound antifreeze proteins by subzero nanoscopy

Abstract

Ice-binding by antifreeze proteins (AFPs) reduces freezing temperatures and arrests ice-crystal ripening, making AFPs essential for survival in ice-laden environments and attractive as biocompatible antifreezes. Whilst their activity was identified over 50 years ago, the physical mechanisms are still debated because experimental insights at the molecular scale remain elusive. Here we introduce optical nanoscopy to resolve the ice/water interfacial dynamics of single AFPs. Using this method, we demonstrate pinning of individual proteins. Surprisingly, this quasi-permanent pinning is lost when freezing point depression activity is inhibited by a single mutation in the ice-binding site. These findings provide direct experimental evidence for the adsorption-inhibition paradigm, pivotal to all theoretical descriptions of activity and offer new insights in the molecular mechanisms by which these biological antifreezes function.