Structural and functional characterization of a C‐type lectin‐like antifreeze protein from rainbow smelt (Osmerus mordax)

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Abstract
Antifreeze proteins (AFPs) are produced by several cold‐water fish species. They depress physiological freezing temperatures by inhibiting growth of ice crystals and, in so doing, permit the survival of these fish in seawater cooler than their normal freezing temperatures. The type II AFP from rainbow smelt (Osmerus mordax), which is a member of the C‐type lectin superfamily, was characterized in terms of its Ca2+‐binding quaternary structure and the role of its single N‐linked oligosaccharide. The protein core of the smelt AFP, shown through sequence homology to be a C‐type lectin carbohydrate‐recognition domain, was found to be protease resistant. Smelt AFP was also shown to bind Ca2+, as determined by ruthenium red staining and a conformational change on Ca2+ binding detected by intrinsic fluorescence. The N‐linked oligosaccharide was found to have no effect on protease resistance, dimerization, or antifreeze activity. Thus its role, if any, in the antifreeze function of this protein remains unknown. Smelt AFP was also shown to be a true intermolecular dimer composed of two separate subunits. This dimerization did not require the presence of N‐linked oligosaccharide or bound Ca2+. Smelt AFP dimerization has implications for the effective solution concentration and measurement of its activity. This finding may also lead to new interpretation of the mechanism of ice‐growth inhibition by this AFP.