Hyperactive antifreeze protein from winter flounder is a very long rod-like dimer of alpha-helices

The winter flounder (Pseudopleuronectes americanus) produces short, monomeric alpha-helical antifreeze proteins (type I AFP), which adsorb to and inhibit the growth of ice crystals. These proteins alone are not sufficiently active to protect this fish against freezing at -1.9 degrees C, the freezing point of seawater. We have recently isolated a hyperactive antifreeze protein from the plasma of the flounder with activity 10-100-fold higher than type I AFP. It is comparable in activity to the AFPs produced by insects, and is capable of conferring freeze resistance to the flounder. This novel AFP has a molecular mass of 16,683 Da and a remarkable amino acid composition that is >60% alanine. CD spectra indicate that the protein is almost entirely alpha-helical at 4 degrees C but partially denatures at 20 degrees C, resulting in a species with a moderately reduced helix content that is stable at up to 50 degrees C. This transformation correlates with irreversible loss of activity. Analytical ultracentrifugation (sedimentation velocity and equilibrium) indicates that the predominant species in solution is dimeric (molecular weight, 32,275). Size-exclusion chromatography reveals a 2-fold higher apparent molecular weight suggesting that this molecule has an unusually large Stokes radius. The axial ratio of the dimer calculated from the sedimentation velocity data is 18:1, confirming that this protein has an extraordinarily long, rod-like structure, consistent with a novel dimeric alpha-helical arrangement. The structural model that best fits these data is one in which the approximately 195 amino acids of each monomer form one approximately 290-A long alpha-helix and associate via a unique dimerization motif that is distinct from that of the leucine zipper and any other coiled-coil.