The finding1 that the turnover of the anionic phospholipid, phosphatidylinositol (PI), increases with increased synaptic activity has long stimulated interest in the possible functional roles of this phospholipid and its phosphorylated products, phosphatidylinositol 4,5-biphosphate (triphosphoinositide or TPI) and phosphatidyl-myoinositol 4-phosphate (diphosphoinositide or DPI)2. TPI metabolism is altered by a variety of cellular stimuli2, but no specific function of the lipid has been identified. However, in many instances (see, for example, refs 3-6) a correlation has been observed between changes in PI metabolism and membrane structure. We have recently correlated membrane macro-viscosity with the lateral mobility of membrane glycoproteins7, and have shown that 2,3-diphosphoglycerate (2,3-DPG), which is similar in structure to TPI, increases glycoprotein lateral mobility8. We now report that TPI also increases the lateral mobility of glycoproteins when added to erythrocyte membranes, and suggest that TPI acts similarly to other polyanions by disrupting the erythrocyte membrane skeleton.
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