Membrane assembly studied by spin-label electron spin resonance.

Conventional electron spin resonance (ESR) of spin-labelled lipids and saturation transfer ESR of spin-labelled proteins are used to study lipid-protein interactions and the mobility of integral proteins, respectively, both in biological membranes and in reconstituted lipid-protein systems. Conventional ESR spectra reveal two spin-labelled lipid populations, the mobility of one of which is hindered by direct interaction with the integral membrane proteins. The proportion of the latter component increases with increasing protein content and with increasing selectivity of the lipid species for the protein. The two-component spectra are quantitated by spectral subtraction and addition, and by simulation using the exchange-coupled Bloch equations. Lipid exchange rates at the protein interface obtained by simulation are found to be consistent with fast exchange found by 2H NMR on similar systems and to reflect the lipid selectivity observed by ESR. Protein-reactive covalent spin labels have been used to study the rotational diffusion and aggregation states of membrane proteins via saturation transfer ESR. The integral protein rotation is uniaxial to the first approximation and the anisotropic motion is analyzed to obtain the principal component of the diffusion tensor. The latter is sensitively dependent on the cross-sectional dimensions of the protein in the membrane, and hence on its state of assembly. A variety of new experiments based on the power saturation properties of the spin-labelled components are also used to determine lipid exchange rates, protein translational diffusion rates, and the location and penetration of proteins in membranes.