Differential scanning calorimetry of the plasma membrane-enriched fraction in Chara

Abstract

Membrane transport phenomena often show discontinuities with temperature. Such non-monotonous thermal behavior was also observed in the plant cell experimental model, the giant internodal cell of the freshwater alga Chara. Employing an NMR relaxation measurement technique in the present study local minima around 15 and 30°C were observed for water efflux in nodal cells of Chara gymnophylla. To test the origin of the observed thermal behavior of membrane transport in C. gymnophylla differential scanning calorimetry (DSC) was used to study thermal transitions in the isolated plasma membrane-enriched fraction and its lipid extract. Three thermal transitions were observed: one at higher temperatures (T(m) = 65°C) irreversible, presumed to be of protein origin, and two reversible resembling lipid phase transitions. The latter transitions were also present in the membrane lipid fraction. Their characteristic temperatures (15-20°C, and 30- 35°C), were close to the ones for the discontinuities in membrane transport. The absence of membrane proteins (in the lipid extract) or their physico- chemical state affected the two lipid transitions in a complex way. From these interactions the type and role of protein-lipid complexes in membrane transport was inferred.