Abstract
Apoplastic extracts of cold-acclimated winter rye (Secale cereale L. cv Musketeer) leaves were previously shown to exhibit antifreeze activity. The objectives of the present study were to identify and characterize individual antifreeze proteins present in the apoplastic extracts. The highest protein concentrations and antifreeze activity were obtained when the leaf apoplast was extracted with ascorbic acid and either CaCl2 or MgSO4. Seven major polypeptides were purified from these extracts by one-dimensional sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis under nonreducing conditions. The five larger polypeptides, of 19, 26, 32, 34, and 36 kD, exhibited significant levels of antifreeze activity, whereas the 11- and 13-kD polypeptides showed only weak activity. Five of these polypeptides migrated with higher apparent molecular masses on SDS gels after treatment with 0.1 M dithiothreitol, which indicated the presence of intramolecular disulfide bonds. The apparent reduction of the disulfide bonds did not eliminate antifreeze activity in four of the polypeptides that contained intramolecular disulfide bonds and exhibited significant levels of antifreeze activity. The amino acid compositions of these polypeptides were similar in that they were all relatively enriched in the residues Asp/Asn, Glu/Gln, Ser, Thr, Gly, and Ala; they all lacked His, except for the 26-kD polypeptide, and they contained up to 5% Cys residues. These polypeptides were examined with antisera to other cystine-containing antifreeze proteins from fish and insects, and no common epitopes were detected. We conclude that cold-acclimated winter rye leaves produce multiple polypeptides with antifreeze activity that appear to be distinct from antifreezes produced by fish and insects.