Functional Analysis of a Bacterial Antifreeze Protein Indicates a Cooperative Effect between Its Two Ice-Binding Domains

Antifreeze proteins make up a class of ice binding proteins (IBPs) that are possessed and expressed by certain cold-adapted organisms to enhance their freezing tolerance. Here we report the biophysical and functional characterization of an IBP discovered in a bacterium recovered from a deep glacial ice core drilled at Vostok Station, Antarctica (IBPv). Our study showed that the recombinant protein rIBPv exhibited a thermal hysteresis of 2 degrees C at concentrations of >50 mu M, effectively inhibited ice recrystallization, and enhanced bacterial viability during freeze thaw cycling. Circular dichroism scans indicated that rIBPv mainly consists of beta strands, and its denaturing temperature was 53.5 degrees C. Multiple-sequence alignment of homologous IBPs predicted that IBPv contains two ice-binding domains, a feature unique among known IBPs. To examine functional differences between the IBPv domains, each domain was cloned, expressed, and purified. The second domain (domain B) expressed greater ice binding activity. Data from thermal hysteresis and gel filtration assays supported the idea that the two domains cooperate to achieve a higher ice binding effect by forming heterodimers. However, physical linkage of the domains was not required for this effect.