What are ice binding proteins and what are they for?
Ice-binding proteins (IBPs) are a group of proteins that help organisms survive in icy environments. Ice is a major threat to living creatures because growth of ice crystals leads to rupture of cellular membranes and to dehydration from loss of water inside the cells.
Unprotected cells and tissues can be severely damaged by these processes. In cold habitats where a major part of the surrounding medium becomes frozen, organisms that need to make it through the winter produce IBPs that help them prevent ice injuries, either by depressing ice growth (freeze-avoidant) or by enduring it (freeze-tolerant).
How do they work?
The unique feature of IBPs is that their ligand is ice, which means that they are evolved for binding water in the frozen state, in an aqueous environment. It is amazing that they can “recognize and bind” an ice crystal, which is an ordered array of water molecules with an elusive surface, when everything around is also water molecules, just less ordered, and they do it very efficiently.
Most other water-soluble proteins have an internal hydrophobic core and a relatively hydrophilic surface. In contrast, IBPs contain a partly hydrophobic patch on one side of their surface. In many cases, the chemical groups of this patch are highly ordered, and water molecules arranged on it create a tiny ice structure that in turn can bind ice efficiently. By directly binding to ice surfaces, IBPs prevent ice crystals from growth or restructuring, processes that if unchecked would lead to cellular damage.
What’s special about the ice binding protein of Antarctic bacteria?
While most IBPs help organisms avoid ice injuries by preventing ice growth and restructuring, the IBP from the Antarctic bacterium Marinomonas primoryensis helps the bacterium adhere to ice crystals. This striking phenomenon is the focus of our paper. Yet, the bacterium itself is not frozen in the binding process or after. In fact, although M. primoryensis favours living at temperatures around 0 °C, freezing harms it.
We think that the bacterium adheres to ice in order to keep its location close to the top layers of the water in the Antarctic lakes, since oxygen and nutrients are more available there as compared to deep water.
Why is the protein so big when only 2% is used for anti-freeze?
The size of this bacterial IBP protein is extremely large, over 1.5 MDa, over 100-fold larger than typical insect and fish IBPs. It is also extremely long and thin with the ice-binding part (2% of the protein mass), located close to the tip farthest from the other end that is attached to the bacteria.
When the protein binds to ice, the long thread-like central section separates the body of the bacterium from touching the ice crystal. This way the bacterium hangs on the ice crystal without freezing into it. The exceptionally long structure of this protein enables the binding to ice while keeping the bacterium in solution assuring that the bacterium is safe from being engulfed in the ice.
What did you discover about Antarctic IBPs in this paper?
With our special equipment and microscopy tools in the lab, we were able to follow the behavior of the bacteria close to the freezing point with and without ice. We showed clearly that the bacteria bind to ice when freely swimming in solution, and that this binding is not exactly like other known IBPs since the bacteria chase the ice when it is slowly melting. We showed that the binding can be blocked by adding a specific antibody that blocks the ice-binding part of the protein. Antibodies that bind other parts of the protein fail to block the binding of the bacteria to ice.
IBPs exist in a variety of organisms that need to survive in cold habitats, including fish, insects, plants, fungi and a range of microorganisms. Still, the use of IBPs as a method to bind the organism to ice is unique to Marinomonas primoryensis and was not found so far in any other species or bacteria.
Thanks to Maya Bar-Dolev, Peter L Davies, and Ido Braslavsky for answering our icy questions. The full paper: ‘Putting life on ice: bacteria that bind to frozen water‘ is open access and now available to read online.