Activity of a two-domain antifreeze protein is not dependent on linker sequence

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 °C at concentrations of >50 μM, effectively inhibited ice recrystallization, and enhanced bacterial viability during freeze-thaw cycling. Circular dichroism scans indicated that rIBPv mainly consists of β strands, and its denaturing temperature was 53.5 °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.

Utilizing avidity to improve antifreeze protein activity: a type III antifreeze protein trimer exhibits increased thermal hysteresis activity

Antifreeze proteins (AFPs) are ice growth inhibitors that allow the survival of several species living at temperatures colder than the freezing point of their bodily fluids. AFP activity is commonly defined in terms of thermal hysteresis, which is the difference observed for the solution freezing and melting temperatures. Increasing the thermal hysteresis activity of these proteins, particularly at low concentrations, is of great interest because of their wide range of potential applications. In this study, we have designed and expressed one-, two-, and three-domain antifreeze proteins to improve thermal hysteresis activity through increased binding avidity. The three-domain type III AFP yielded significantly greater activity than the one- and two-domain proteins, reaching a thermal hysteresis of >1.6 °C at a concentration of <1 mM. To elucidate the basis of this increase, the data were fit to a multidomain protein adsorption model based on the classical Langmuir isotherm. Fits of the data to the modified isotherms yield values for the equilibrium binding constants for the adsorption of AFP to ice and indicate that protein surface coverage is proportional to thermal hysteresis activity.

Conjugation of type I antifreeze protein to polyallylamine increases thermal hysteresis activity

Antifreeze proteins (AFPs) are ice binding proteins found in some plants, insects, and Antarctic fish allowing them to survive at subzero temperatures by inhibiting ice crystal growth. The interaction of AFPs with ice crystals results in a difference between the freezing and melting temperatures, termed thermal hysteresis, which is the most common measure of AFP activity. Creating antifreeze protein constructs that reduce the concentration of protein needed to observe thermal hysteresis activities would be beneficial for diverse applications including cold storage of cells or tissues, ice slurries used in refrigeration systems, and food storage. We demonstrate that conjugating multiple type I AFPs to a polyallylamine chain increases thermal hysteresis activity compared to the original protein. The reaction product is approximately twice as active when compared to the same concentration of free proteins, yielding 0.5 °C thermal hysteresis activity at 0.3 mM protein concentration. More impressively, the amount of protein required to achieve a thermal hysteresis of 0.3 °C is about 100 times lower when conjugated to the polymer (3 μM) compared to free protein (300 μM). Ice crystal morphologies observed in the presence of the reaction product are comparable to those of the protein used in the conjugation reaction.

Two domains of RD3 antifreeze protein diffuse independently

Antifreeze proteins (AFPs) make up a class of structurally diverse proteins that help to protect many organisms from freezing temperatures by inhibiting ice crystal growth at temperatures below the colligative freezing point. AFPs are typically small proteins with a relatively flat, slightly hydrophobic binding region that matches the lattice structure of a specific ice crystal plane. The only known two-domain AFP is RD3 from the Antarctic eel pout. It consists of two nearly identical type III domains connected by a nine-residue linker. This protein exhibits higher activity than the single-domain protein at low concentrations. The initial solution structure of RD3 revealed that the domains were aligned so that the binding regions were nearly coplanar, effectively doubling the surface area for binding. A more recent report suggests that the domains may not be aligned in solution but rather diffuse independently. To resolve the issue, we have measured the NMR residual dipolar couplings using alignment media of stretched gels and filamentous phage to determine the relative orientation of the domains. We find that the two domains of RD3 are free to move relative to each other, within the constraint of the flexible nine-residue linker. Our data show that there is no strongly preferred alignment in solution. Furthermore, the flexibility and length of the linker are sufficient to allow the two domains to have their binding faces in the same orientation and coplanar for simultaneous binding to an ice crystal surface.