PURIFICATION, CHARACTERIZATION, AND STRUCTURAL-ANALYSIS OF A PLANT LOW-TEMPERATURE-INDUCED PROTEIN

We have purified to near homogeneity a recombinant form of the protein BN28 (rBN28), expressed in response to low temperature in Brassica na pus plants, and we have determined its solution structure. Antibodies raised against rBN28 were used to characterize the recombinant and nat ive proteins. Similar to many other low-temperature-induced proteins, BN28 is extremely hydrophilic, such that it remains soluble following boiling. Immunoblot analysis of subcellular fractions indicated that B N28 was not strongly associated with cellular membranes and was locali zed exclusively within the soluble fraction of the cell. Contrary to p redicted secondary structure that suggested significant helical conten t, circular dichroism analysis revealed that rBN28 existed in aqueous solution largely as a random coil. However, the helical propensity of the protein could be demonstrated in the presence of trifluoroethanol. Nuclear magnetic resonance analysis further showed that rBN28 was in fact completely unstructured (100% coil) in aqueous solution. Although it had earlier been speculated that BN28-like proteins from Arabidops is thaliana might possess antifreeze protein activity (S. Kurkela and M. Franck [1930] Plant Mol Biol 15: 137-144), no such activity could b e detected in ice recrystallization assays with rBN28.