The effect of exogenous glycerophospholipids on the fluorescence polarization ratios of Escherichia coli cells labelled with diphenylhexatriene

Abstract

Saturated, unsaturated, and short acyl chain analogues of phosphatidylcholine and phosphatidylcholine and phosphatidylethanolamine were incorporated into a deep heptoseless mutant of Escherichia coli, strain D21F2, and into the parent wild-type strain, K12. Normal and lipid-treated cells or lipid extracts from such cells were labelled with diphenylhexatriene and their fluorescence polarization ratios were measured as a function of temperature. Incorporations of dipalmitoyl analogues of phosphatidylethanolamine and/or phosphatidylcholine in the presence of Ca 2+ caused an increase in polarization ratios over a wide temperature range and the appearance of new phase transitions at 25–30°C as measured in whole D21F2 cells. Incorporation into D21F2 of the dioleoyl analogues of these glycerophospholipids under similar conditions had the opposite effect on the polarization ratios and, in the case of dioleoylphosphatidylethanolamine, caused the occurrence of a new phase transition at 20°C. Incorporation of these same lipids in K12 cells, in the presence of Ca 2+, caused changes in the polarization ratios similar to those recorded for D21F2 cells when measurements were made on whole cells. Furthermore incorporation of didecanoyl-phosphatidylcholine in wild-type cells, in the presence of Ca 2+, substantially decreased the polarization ratio and broadened the phase transition as could be measured with cell preparations. Since Ca 2+ stimulates incorporation of lipid, the changes in polarization ratio were always greater when cells had been exposed to exogenous lipid in the presence of this cation. However, even in cells not treated with lipid, Ca 2+ caused increases in the polarization ratio and affected the thermotropic structural transitions. The polarization ratios of extracted lipids were always reduced when compared to whole cells. Generally there was an attenuation of any differences in polarization ratio between normal and glycerophospholipid-treated samples. Extracted lipids also displayed broadened phase transitions. The results as a whole indicated that E. coli cells respond to the uptake of lipid and to the presence of Ca 2+ by changes in their thermotropic mesomorphic behaviour. These changes reflect to a large extent the fluidity of the incorporated lipid and are exerted on a structural system the phase transitions of which are strongly influenced by the presence of non lipid components in the membrane.