Lipid-protein interactions in rat renal subcellular membranes: A biophysical and biochemical study

The phase behavior of plasma membrane (PM), endoplasmic reticulum (ER), and nuclear membranes (NM) isolated from adult rat papillary cells was studied using the molecular probe Laurdan. The steady-state fluorescence data anal ysis was correlated with the lipid composition obtained by biochemical assa ys. The comparison between intact membranes and protein-free reconstituted vesicles using the whole lipid extract shows the essential role of proteins on the temperature response of natural membranes. The phospholipid (PL) an d cholesterol (Cho) content was measured in the three membrane fractions, t he PW Cho molar ratio being between 1.5 and 1.9. However, Laurdan's paramet ers in NM show a fluid phase state pattern even at low temperature (5 degre es C), with a restricted dipole relaxation in comparison with that displaye d in liquid crystalline phase state lipid model membranes. PM and ER are in a gel-like state at temperatures below 20 degrees C, showing increasing di pole relaxation with temperature. The curved fits obtained are characterist ic of cholesterol-enriched membranes. The distinctive phase behavior of nuc lear membranes vanishes when proteins are extracted. However, relaxation is still faster in this fraction, which correlates with the native lipid comp osition. NM has the lowest percentage of phosphatidylinositol and sphingomy elin-the latter being a highly saturated phospholipid-and the highest perce ntage of phosphatidylcholine and phosphatidylethanolamine (PE), nuclear PE being enriched in arachidonic acid. All these changes agree with the higher fluidity of NM compared with ER or PM in the conditions assayed. (C) 2000 Academic Press.