Enhancement of insect antifreeze protein activity by solutes of low molecular mass

Antifreeze proteins (AFPs) lower the non-equilibrium freezing point of water (in the presence of ice) below the melting point, thereby producing a difference between the freezing and melting points that has been termed thermal hysteresis. In general, the magnitude of the thermal hysteresis depends upon the specific activity and concentration of the AFP. This study describes several low-molecular-mass solutes that enhance the thermal hysteresis activity of an AFP from overwintering larvae of the beetle Dendroides canadensis. The most active of these is citrate, which increases the thermal hysteresis nearly sixfold from 1.2 degrees C in its absence to 6.8 degrees C. Solutes which increase activity approximately fourfold are succinate, malate, aspartate, glutamate and ammonium sulfate. Glycerol, sorbitol, alanine and ammonium bicarbonate increased thermal hysteresis approximately threefold. Interestingly, 0.5 mol l-1 sodium sulfate eliminated activity. Solute concentrations between 0.25 and 1 mol l-1 were generally required to elicit optimal thermal hysteresis activity. Glycerol is the only one of these enhancing solutes that is known to be present at these concentrations in overwintering D. canadensis, and therefore the physiological significance of most of these enhancers is unknown. The mechanism(s) of this enhancement is also unknown. The AFP used in this study (DAFP-4) is nearly identical to previously described D. canadensis AFPs. The mature protein consists of 71 amino acid residues arranged in six 12- or 13-mer repeats with a consensus sequence consisting of Cys-Thr-X3-Ser-X5-X6-Cys-X8-X9-Ala-X11-Thr-X1 3, where X3 and X11 tend to be charged residues, X5 tends to be Thr or Ser, X6 to be Asn or Asp, X9 to be Asn or Lys and X13 to be Ala in the 13-mers. DAFP-4 is shorter by one repeat than previously described D. canadensis AFPs.