Differences in fluidity between bilayer halves of tumour cell plasma membranes

Abstract

SURFACE membranes of both normal and transformed cells appear to have an asymmetric distribution of phospholipids across the bilayer^1–6. The potential role of this distribution in platelet aggregation and in pathological conditions such as sickle cell anaemia and cell transformation has been investigated^7–9. However, neither the structural nor functional significance of this asymmetrical distribution is understood at present. As the lipid composition of the two sides of the plasma membrane bilayer is different it seems possible that the ‘fluidity’ or other physicochemical properties are also different. It has been shown that the neutral zwitterionic lipids (phosphatidylcholine and sphingomyelin) are located primarily in the outer monolayer while the anionic zwitterionic lipids (phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol) are located primarily in the inner or cytoplasmic monolayer. In model systems the former lipids are usually more fluid while the latter lipids are more rigid¹⁰. Thus, a fluidity gradient could exist across the bilayer of biological membranes. Such a gradient and alterations of it may have important consequences in vectorial processes such as small molecule transport, receptor mobility, hormone receptor interactions and the activity of membrane-bound enzymes. However, as mammalian cells can compensate or homeoviscously adapt and because biological membranes are complex mixtures of cholesterol and phospholipids with different fatty acyl chain length or unsaturation, direct experimental measurement is necessary to determine if such vertical asymmetry of ‘fluidity’ exists across the plasma membrane bilayer of mammalian cells^11–16. I report here use of a fluorescent probe molecule, β-parinaric acid, used in conjunction with the amino labelling reagent 2,4,6-trinitrobenzenesulphonic acid (TENS) to investigate this asymmetry. The data obtained are consistent with the conclusion that the inner monolayer of the plasma membrane is less fluid than the outer monolayer.