PLANT PHYSIOLOGY , Vol 111, Issue 1 313-327, Copyright © 1996 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
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
M. Uemura, S. J. Gilmour, M. F. Thomashow and P. L. Steponkus
Department of Soil, Crop and Atmospheric Sciences, Cornell University, Ithaca, New York 14853 (M.U., P.L.S.)
Several cold-regulated (COR) polypeptides, which have little or no amino
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 was 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 phosphatidylcholine
(either 1-palmitoyl-2-oleoyl-, dioleoyl-, or
dilinoleoylphosphatidylcholine), a mixture of dioleoylphosphatidylcholine,
dioleoylphosphatidylethanolamine, and free sterols (1:1:1, mol:mol), or the
total lipid extract of the plasma membrane of either nonacclimated or
cold-acclimated rye leaves. When the SUVs were suspended in a dilute
tris(hydroxymethyl)-aminomethane/2-(N-morpholino)ethanesulfonic acid
buffer, both COR6.6 and COR15am invariably decreased the incidence of
freeze-induced fusion regardless 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. Moreover, the COR polypeptides did not decrease the incidence of
freeze-induced leakage[mdash]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 increase
in freeze-induced leakage. When considered collectively, these results
suggest that neither COR6.6 nor COR15am has a direct cryoprotective effect
on SUVs frozen in vitro.
