Effects of COR6.6 and COR15am polypeptides encoded by COR (cold-regulated) genes of Arabidopsis thaliana on dehydration-induced phase transitions of phospholipid membranes.

Cold acclimation of Arabidopsis thaliana includes the expression of cold-regulated (COR) genes and the accumulation of COR polypeptides. The hydration characteristics of two COR polypeptides, COR6.6 and COR15am, have been determined and their effects on the dehydration-induced liquid crystalline-to-gel and lamellar-to-hexagonal II phase transitions in phospholipid mixtures have been examined. After dehydration at osmotic pressures between 8 and 150 MPa, the water content of the COR polypeptides was less than that of bovine serum albumin, with COr15am the least hydrated: bovine serum albumin > COR6.6 > COR15am. Neither COR6.6 nor COR15am altered the dehydration-induced gel lamellar --> fluid lamellar phase transition temperature of either dipalmitoylphosphatidylcholine or dioleoylphosphatidylcholine (DOPC). In multilamellar vesicles of dioleoylphosphatidylethanolamine:DOPC (1:1, mol:mol) prepared by either freeze-thaw or reverse-phase evaporation methods, neither COR6.6, COR15am, nor bovine serum albumin altered the incidence of the dehydration-induced formation of the inverted hexagonal phase as a function of osmotic pressure. However, a specific ultrastructural alteration--the formation of a striated surface morphology in the lamellar domains--was observed in mixtures of dioleoylphosphatidylethanolamine:DOPC that were dehydrated in the presence of COR15am. Nevertheless, neither COR6.6 nor COR15am appears to participate in a specific protein-phospholipid interaction that alters the dehydration-induced phase behavior of phospholipid vesicles.