Ganglioside-protein interactions: spin-label electron spin resonance studies with sodium-potassium ATPase membranes

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Abstract
Lipid-protein interactions in (Na+,K+)-ATPase-rich membranes from Squalus acanthias have been studied using spin-labeled derivatives of the mono- and disialogangliosides GM1, GM2, GM3, and GD1b, in conjunction with electron spin resonance (ESR) spectroscopy. Ganglioside-protein interactions are revealed by the presence of a second component in the ESR spectra of the membranes in addition to a component that corresponds closely to the ESR spectra obtained from dispersions of the extracted membrane lipids. This second component corresponds to spin-labeled gangliosides whose chain motion is significantly restricted relative to that of the fluid lipids in the membrane or the lipid extract. A small selectivity for the motionally restricted component associated with the protein is found in the order GD1b > GM1 .apprxeq. GM2 .apprxeq. GM3. Comparison with previous results from spin-labeled phospholipids in the same system [Esmann, M., Watts, A., and Marsh, D. (1985) Biochemistry 24, 1386-1393] shows that the spin-labeled monosialogangliosides GM1, GM2, and GM3 display little selectivity in the lipid-protein interaction relative to spin-labeled phosphatidylcholine. The spectral characteristics of both the fluid and motionally restricted spin-label components differ very significantly, however, between the gangliosides and the phospholipids. The outer hyperfine splitting of the motionally restricted component is smaller for the gangliosides than for the phospholipids, indicating a smaller degree of motional restriction on interaction of the ganglioside lipid chains with the protein. The effective order parameters of the fluid component are larger for the gangliosides than for the phospholipids, consistent with a location of the spin-labeled segment closer to the polar interface of the membrane in the case of the gangliosides.