| Abstrakt: |
It has recently been shown that some galactose-specific plant lectins protect thylakoid membranes from freeze–thaw damage [D. K. Hinchaet al. Plant Physiol.103,59–65 (1993)]. In the present investigation, theRicinus communisseed lectins RCA60and RCA120have been used to investigate further the effects of lectin binding on the physical properties of thylakoids. Both proteins bind to digalactolipids, but only RCA60is cryoprotective. We found that RCA60reduced the glucose permeability of the membranes. Fluorescence depolarization measurements with the probe trimethylammonium-diphenylhexatriene (TMA-DPH) showed a significant reduction of lipid fluidity in the presence of RCA60, but not with RCA120. The fluidity of the hydrocarbon core region of thylakoids, as probed with DPH, was not influenced. These results were corroborated by electron spin resonance spectroscopy using doxyl stearic acids (DSA) with the reporter group located in position 5, 12, or 16 of the fatty acyl chains. Clear effects of RCA60were only apparent with 5DSA. When we used the fluorescence probe merocyanine 540 to investigate the lipid packing density in the glycerol backbone region of the membrane, we found a decrease in fluorescence emission in the presence of RCA60. We conclude from these data that cryoprotective lectins lead to changes in the packing of the membrane lipids in the interfacial region which result in a lower solute permeability of the membranes. |
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