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 co ld-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 li quid crystalline-to-gel and lamellar-to-hexagonal II phase transitions in phospholipid mixtures have been examined. After dehydration at osm otic pressures between 8 and 150 MPa, the water content of the COR pol ypeptides 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 dipalmitoylphosphatidy lcholine or dioleoylphosphatidylcholine (DOPC). In multilamellar vesic les of dioleoylphosphatidylethanolamine:DOPC (1:1, mol:mol) prepared b y either freeze-thaw or reverse-phase evaporation methods, neither COR 6.6, COR15am, nor bovine serum albumin altered the incidence of the de hydration-induced formation of the inverted hexagonal phase as a funct ion of osmotic pressure. However, a specific ultrastructural alteratio n-the formation of a striated surface morphology in the lamellar domai ns-was observed in mixtures of dioleoylphosphatidylethanolamine:DOPC t hat were dehydrated in the presence of COR15am. Nevertheless, neither COR6.6 nor COR15am appears to participate in a specific protein-phosph olipid interaction that alters the dehydration-induced phase behavior of phospholipid vesicles.