Adsorption of fungal ice-binding proteins to whole ice crystal surface is deficient for high thermal hysteresis activity but strongly inhibits ice recrystallization

Many psychrophilic microorganisms synthesize ice-binding proteins (IBPs) for their cold-survival. The function of IBP is evaluated by its effect on the nonequilibrium water freezing-point depression called “thermal hysteresis (TH)” and inhibitory effect on the growth of numerous ice crystals called “ice recrystallization inhibition (IRI)”. To obtain mechanical insight of the two activities, here we developed a modified method of ice affinity purification and extracted two new IBP isoforms from Arctic glacier fungus Psychromyces glacialis . It was found that one isoform is approximately 25 kDa protein (PsgIBP_S), while the other is a 28 kDa larger protein (PsgIBP_L) that constructs an intermolecular dimer. Their TH activities were less than 1 °C at millimolar concentrations, implying that both isoforms are designated as the moderately active but not the hyperactive IBP species. It further appeared that both isoforms exhibit high ice-recrystallization inhibition (IRI) activity even at sub-micromolar concentrations. Furthermore, they are capable of binding to whole surface of a hemispherical single ice crystal, although such manner of ice-binding was generally observed for hyperactive IBP species. These results suggest that IBP’s binding ability to whole ice crystal surface is deficient for the hyperactivity but is crucial for the significant IRI activity.