Solution structure of hyperactive type I antifreeze protein

Antifreeze proteins (AFPs) protect freeze-intolerant fish species living in icy polar waters against freeze damage. A 34 kDa dimeric type I antifreeze protein (wfAFP 1h) with unusually high activity in comparison to all other antifreeze proteins in fish was recently discovered in the winter flounder. We have measured the size and shape of this hyperactive AFP by using small angle X-ray scattering. Our results show that wfAFP 1h adopts a long cylindrical shape with a length of 19 +/- 2 nm and a diameter of 2.3 +/- 0.2 nm, which means that wfAFP 1h does not form a fully extended helical dimer in solution. These findings call for a revision of the structural model of wfAFP 1h and the concept of a flat, threonine-rich ice-binding site extending down the length of the protein. Instead, the hyperactive nature of wfAFP 1h may be derived from a unique 3D arrangement of the helices -yet to be resolved-by which it is able to bind to ice surfaces