Use of a fluorescent probe to determine the viscosity of LM cell membranes with altered phospholipid compositions

Abstract

The phospholipid composition of [mouse] LM cells grown in tissue culture was altered by substituting ethanolamine for choline in the growth medium. The plasma membrane isolated from cells grown in medium containing ethanolamine for 83 h had a 6 fold increase in the ratio of phosphatidylethanolamine to phosphatidylcholine, the 2 major phospholipid classes. This was accompanied by small changes in other lipid components of the membrane. There was also a 6-fold increase in the amount of triacylglycerols and alkyldiacylglycerols which were not associated with the membrane fraction of the cell. No significant changes occurred in the lipid composition of cells during growth in choline containing medium. The viscosity of plasma membranes was studied in whole cells and isolated membranes using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene. Plasma membranes isolated from ethanolamine-supplemented cells had greater viscosities than membranes isolated from choline-supplemented cells. When whole cells were labeled with the fluorescent probe, the opposite trend in the apparent membrane viscosity was observed. This was due primarily to the probe penetrating into non-membranous neutral lipids rather than remaining localized in the surface membrane of the cells. Since the enthanolamine-supplemented cells contained more low viscosity neutral lipids, whole cells gave an apparently lower viscosity as compared with choline-supplemented cells. Measurements carried out on whole cells gave an inaccurate determination of the viscosity of the surface membrane.