Fluorescence studies and the nature of ion channels.

spectra from the membranes are very similar to those of aqueous bilayers of the extracted lipids. Protein-lipid interactions are revealed in this system by the sensitivity of membrane-associated enzymic activities to structural transitions detected in the lipid-bilayer component of the membrane. The cytochrome oxidase complexes are a convenient experimental model in which it is possible to vary the 1ipid:protein ratio and hence distinguish between specific protein-lipid interactions as distinct from protein-‘crowding’ effects on the lipid bilayer. The e.s.r. spectra of the complexes reveal a two-component spectrum arising from a strongly immobilized population of lipid interacting directly with the protein, together with a fluid-bilayer component whose motional characteristics are strongly perturbed at high protein: lipid ratios. The rod outersegment and acetylcholine-receptor membranes are non-reconstituted systems that contain a high proportion of a single protein and are thus appropriate for studying specific lipid-protein interactions. A strongly immobilized, protein-associated lipid component is observed in the spin-label spectra from both membranes, although they differ considerably in their degree of fluidity and apparent protein crystallinity. This suggests that a specifically immobilized, boundary-lipid region may be a general feature of lipid-protein interactions in biological membranes.