|Crystal structure of Rhagium inquisitor antifreeze proteins. A, overall fold of RiAFP. β-strands are shown in blue and yellow. The threonine side chains and coordinated water molecules (red) on the IBS are shown in ball-and-stick representation. The disulfide bond is indicated in green. B, secondary structure diagram with sequentially numbered ice-binding β-strands in blue, hydrophilic β-strands in yellow. N and C represent the protein termini. C and D, flat ice-binding surface of RiAFP (C) and TmAFP (D), represented by a hydrophobic surface diagram produced in chimera, which relies on the Kyte-Doolittle scale to rank amino acid hydrophobicity. Water molecules coordinated by the threonine hydroxyls on the IBS are shown in red. E, RiAFP dimers in the crystallographic asymmetric unit, with the ice-binding surfaces packed face to face.
Creatures that manage to survive extremely cold conditions do so by employing a number of clever behavioral and biological strategies. Some vertebrates, plants, fungi and bacteria produce antifreeze proteins, which bind to and prohibit the growth of intracellular ice crystals that otherwise would lead to the organisms’ demise.
In a recent paper in The Journal of Biological Chemistry, researchers at Yale University and Queen’s University reported the crystal structure of the most potent antifreeze protein known — RiAFP, found in a longhorned beetle. Though Rhagium inquisitor isn’t exactly a pretty insect, it is pretty tough: It can survive temperatures as low as −25°C in Siberia by synthesizing RiAFP.
Using X-ray crystallography, the research team determined the RiAFP structure contains 1,914 non-hydrogen protein atoms, with no atoms heavier than sulfur by ab initio direct methods, revealing a β-solenoid structure with polypeptide chains that contain capping structures that reverse the handness (from left to right or vice versa) of the strands at the end terminus. These structures prevent end-to-end interactions that lower the solubility of RiAFP and lead to oligomerization and aggregation.
RiAFP is the first antifreeze protein isolated that can be produced in large amounts (up to 50 milligrams/unit). The authors say that the “hyperactivity and efficient recombinant production” makes it a good candidate for future experiments that will explore the protein’s freeze resistance, control of ice growth, and form and structure.
Antifreeze proteins of different, unrelated species have been explored for many years. They illustrate the concept of convergent evolution — when unrelated species develop similar traits to adapt to their environments. Knowing more about how antifreeze proteins work could improve our ability to engineer freeze-tolerant crops and raise more hardy fish. Some makers of consumer products have even dreamed up nature-inspired, if not necessarily cheap, goods like Crème de la Mer’s Lip Balm ($50 at department stores), which is said to be infused with a marine antifreeze protein to protect your pout from even the harshest weather.