29. Ice-binding proteins and their interactions with ice crystals

Abstract

Ice-binding proteins (IBPs) depress the freezing point of the body fluids below the melting point, resulting in a thermal hysteresis (TH) that prevents freezing of the organism in supercooled conditions and inhibits ice re-crystallization in frozen tissues. The potential of these proteins in the medical sector, in cryopreservation, in the frozen food industry, and in agriculture is enormous. The mechanisms by which IBPs interact with ice surfaces are still not completely understood and the potential of IBPs as cryoprotecting agents has not yet been realized. At the molecular level it was found that the IBPs coordinate and stabilize water molecules on their binding surface to form an ice-like water film. While this ice-like film is too small to nucleate supper-cooled water it serves as a mechanism of tight binding to ice. Still, the way these IBPs influence ice growth and the activity differences between difference types of IBPs is the subject of current research. We are investigating the interactions of IBPs with ice surfaces. In particular we are interested in the difference between hyperactive antifreeze proteins and moderately active ones, and the dynamic nature of the protein:ice interaction. We have developed novel methods to study these issues, including fluorescence microscopy techniques combined with temperature-controlled microfluidic devices (Celik et al., PNAS 2013, Drori et al., J.R. Soc. Interface 2014, Drori et al., RSC Adv. 2015). These techniques enable the replacement of the IBP solution surrounding an IBP-bound ice crystal by other solutions, without perturbing the system, which enables us to investigate the dynamic nature of the interactions between IBP and ice. The results show that binding of IBP to ice is irreversible, and that the TH-gap is sensitive to the time allowed for the proteins to accumulate on ice surfaces. This sensitivity changes dramatically between different types of IBPs. In a study of ice shaping during growth and melting we have demonstrated a correlation between ice crystal shapes, the shaping process, and the affinity of IBPs for the basal plane (Bar-Dolev et al., J.R. Soc. Interface 2012). Our results point to a connection between the dynamics and level of activity of different types of IBP to their ability to bind to specific ice orientations, in particularly to the basal plane of the ice. These results contribute to an understanding of the mechanisms by which diverse IBPs act that will be critical for the successful use of IBP in cryobiological applications. Supported by ERC, NSF, ISF, and CIHR.