Purification and composition of a thermal hysteresis producing protein from the milkweed bug, Oncopeltus fasciatus

A protein which produces a thermal hysteresis (a difference between the freezing and melting points) was purified from the hemolymph of the milkweed bug, Oncopeltus fasciatus. The amino acid composition of the Oncopeltus thermal hysteresis protein is somewhat different from that of the larvae of the beetle, Tenebrio molitor, which is the only other insect from which such a protein has as yet been purified. The major difference between the two is the large amount of serine (30.5% of the amino acid residues) and glycine (20.0%) present in the O. fasciatus protein. Both insect proteins have a composition which consists of approximately 60% polar amino acids and lacks large amounts of alanine. In these respects they are quite different from the fish protein antifreezes. The apparent differences in structure of the thermal hysteresis proteins and glycoproteins indicates that these proteins have evolved independently and therefore offer an interesting example of convergent evolution.