Fully active QAE isoform confers thermal hysteresis activity on a defective SP isoform of type III antifreeze protein

Type III antifreeze protein is naturally expressed as a mixture of sulfopropyl-Sephadex (SP) and quaternary aminoethyl-Sephadex (QAE)-binding isoforms, whose sequence identity is approximately 55%. We studied the ice-binding properties of a SP isoform (nfeAFP6) and the differences from those of a QAE isoform (nfeAFP8); both of these isoforms have been identified from the Japanese fish Zoarces elongatus Kner. The two isoforms possessed ice-shaping ability, such as the creation of an ice bipyramid, but nfeAFP6 was unable to halt crystal growth and exhibited no thermal hysteresis activity. For example, the ice growth rate for nfeAFP6 was 1000-fold higher than that for nfeAFP8 when measured for 0.1 mm protein solution at 0.25 degrees C below the melting point. Nevertheless, nfeAFP6 exhibited full thermal hysteresis activity in the presence of only 1% nfeAFP8 (i.e. [nfeAFP8]/[nfeAFP6] = 0.01), the effectiveness of which was indistinguishable from that of nfeAFP8 alone. We also observed a burst of ice crystal growth from the tip of the ice bipyramid for both isoforms on lowering the temperature. These results suggest that the ice growth inhibitory activity of an antifreeze protein isoform lacking the active component is restored by the addition of a minute amount of the active isoform.