EFFECTS OF COR6.6 AND COR15AM POLYPEPTIDES ENCODED BY COR (COLD-REGULATED) GENES OF ARABIDOPSIS-THALIANA ON THE FREEZE-INDUCED FUSION AND LEAKAGE OF LIPOSOMES

Several cold-regulated (COR) polypeptides, which have little or no ami no acid sequence identity with known proteins, are synthesized during cold acclimation of Arabidopsis thaliana. However, the function of the polypeptides has yet to be elucidated. The objective of this study wa s to determine if COR6.6 and COR15am influence the incidence of either freeze-induced fusion or freeze-induced leakage of small unilamellar vesicles (SUVs) composed of either a single species of phosphatidylcho line (either 1-palmitoyl-2-oleoyl-, dioleoyl-, or dilinoleoylphosphati dylcholine), a mixture of dioleoylphosphatidylcholine, dioleoylphospha tidylethanolamine, and free sterols (1:1:1, mol:mol), or the total lip id extract of the plasma membrane of either nonacclimated or cold-accl imated rye leaves. When the SUVs were suspended in a dilute methyl)ami nomethane/2-(N-morpholino)ethanesulfonic acid buffer, both COR6.6 and COR15am invariably decreased the incidence of freeze-induced fusion re gardless of the lipid composition. However, if the SUVs were suspended in a dilute solution of either sucrose or NaCl, the COR polypeptides had little or no effect on the incidence of freeze-induced fusion. Mor eover, the COR polypeptides did not decrease the incidence of freeze-i nduced leakage-regardless of whether the SUVs were suspended in either the dilute buffer alone or with added sucrose or NaCl. In fact, with SUVs composed of a single species of phosphatidylcholine suspended in the dilute buffer, the COR polypeptides resulted in an anomalous incre ase in freeze-induced leakage. When considered collectively, these res ults suggest that neither COR6.6 nor COR15am has a direct cryoprotecti ve effect on SUVs frozen in vitro.