The Influence of Lipid Substitution on Thyrotropin-Receptor Interactions in Artificial Vesicles

Abstract

The fluorescent probes N-1-naphthyl- N-phenylamine (NPN), 12-(9-anthroyl) stearic acid (AS) and 2-(9-anthroyl) palmitic acid (AP) were introduced into a) purified human thyroid membranes, b) artificial vesicles containing solubilized human thyroid membrane proteins, c) purified thyroid stimulating hormone (TSH) receptors, and the temperature dependence of fluorescence polarization was measured spectrofluorometrically. Reduction in the temperature of the reaction, or increase in ionic concentration, imposed a motional constraint on the membrane lipids and reduced or abolished hormone binding. The temperature dependence of fluorescence polarization in human thyroid membranes indicates that a phase transition occurs at 28 C. A corresponding transition in hormone binding occurred at that same temperature. Substitution of dipalmitoyl lecithin (DPL) or dimyristoyl lecithin (DML) into artificial vesicles containing the TSH receptor shifted the temperature- dependent transition of fluorescence polarization and TSH binding to 24 C and 42 C, the melting temperatures of DPL and DML, respectively. TSH binding to receptors in vesicles composed of a mixture of DPL and DML showed a transition at 24 C only. These studies support the view that lipids modulate TSH-receptor interactions and suggest that the TSH receptor proteins segregate in association with the lipid which is in the “fluid” phase.