A molecule that prevents the blood of winter flounder (Pseudopleuronectes americanus) from freezing is the first protein discovered to have a water-filled core.

Peter Davies of Queen's University in Kingston, Canada, and colleagues crystallized a protein called Maxi, which binds ice crystals to prevent larger ice structures from forming. Unlike most proteins, which have hydrophobic inner surfaces that exclude water, the researchers found that Maxi's core is full of water.

The inner surface of each Maxi protein holds about 400 water molecules in ordered structures layered between the protein's chains. This water structure sticks outside the protein, where it appears to bind to ice.

Science 343, 795–798 (2014)