Photoaffinity spin-labeling of the Ca2+ ATPase in sarcoplasmic reticulum: evidence for oligomeric structure

Lipid-protein interactions and the function of the Ca2+-ATPase of sarcoplasmic reticulum

Regardless of the nature of the protein constituents of membranes, the molecular arrangement of lipids interacting with them must satisfy hydrophobic, ionic, and steric requirements. Biological membranes have a great diversity of lipid constituents, and this diversity might have functional roles. It has been proposed, for example, that the hydrophobic regions of membrane proteins are stabilized in the membrane through interactions with lipids able to adopt configurations other than the bilayer structure. Progress in understanding at the molecular level how lipid-protein interactions control the properties of membrane proteins has been hindered by the lack of information concerning the structure of the hydrophobic regions of membrane proteins. Nevertheless, there are many examples in the literature describing how changes in the lipid environment affect physical and biochemical properties of membrane proteins. From these studies, discussed in this review, an overall picture of how lipids and proteins interact in membranes is beginning to emerge.

Spin-label studies of the oligomeric structure of band 3 protein in erythrocyte membranes and in reconstituted systems

A spin-labeled fatty acid (16-doxylstearic acid), linked by an ester bond to a maleimide or a nitrene residue, was covalently attached to band 3 of erythrocyte membranes. The electron spin resonance spectrum of the spin-labeled protein was examined at different temperatures in: (a) whole erythrocyte ghosts; (b) ghosts depleted of spectrin and actin; (c) alkaline-treated ghosts; (d) vesicles made with purified band 3 reassociated with dimyristoylphosphatidylcholine. Most spectra are composite with a major component corresponding to a large overall splitting. The determination of the percentage of the immobilized component was carried out by pairwise subtraction. At low temperatures (1-7 degrees C), the highest fraction of immobilized component was found in dimyristoylphosphatidylcholine vesicles (approx. 100%); alkaline-treated membranes had approx. 75% of the immobilized component at the same temperature; whole erythrocyte, spectrin/actin-depleted and spectrin/actin/ankyrin-depleted ghosts gave identical results (approx. 60% of immobilized component). The immobilized fraction decreased in all samples with increasing temperature or addition of a nonsolubilizing concentration of dodecyl octaethylene glycol monoether. In dimyristoylphosphatidylcholine vesicles, however, the modification in the ration of the two components was obtained only above the lipid transition temperature (23 degrees C). The strong immobilization of the spin-labeled lipid chain at all temperatures suggested trapping of the lipid chain between proteins. At low temperature, in dimyristoylphosphatidylcholine vesicles or in alkaline-treated ghosts, lipid-protein segregation is likely to take place. In whole erythrocyte ghosts, on the other hand, the large contribution of the motionally restricted component at physiological temperature indicates the oligomeric nature of band 3. Partial dissociation of the oligomers occurs as the temperature is increased, but the presence or absence of cytoskeletal proteins has no influence on the state of oligomerization of band 3.

Perturbation of the structure and function of a membranous Ca2+-ATPase by non-solubilizing concentrations of a non-ionic detergent

The present study characterizes the effect of octa(ethyleneglycol)-monododecylether (C12E8) on Ca2+-ATPase membranes, prepared from sarcoplasmic reticulum (SR). At low concentrations C12E8 is incorporated into the membrane (less than or equal to 0.2 g/g protein), without any solubilization or appreciable morphological changes of freeze-fracture replica. Binding studies of C12E8 to ATPase membranes and SR lipid liposomes suggest that the major part of the detergent interacts with lipid. Solubilization of ATPase membranes occurs at a free concentration of C12E8 close to the critical micellar concentration (c.m.c); at low temperatures (2 degrees C) phospholipid is extracted somewhat more easily than ATPase. Electron-spin resonance (ESR) spectra of appropriate spin labels, incorporated into ATPase membranes, show that C12E8 strongly increases the fluidity of the lipid phase and the rotational diffusion of ATPase in the membrane. The effect of C12E8 on the ESR spectra is indistinguishable from that produced by a rise in temperature. Incorporation of C12E8 alters the functional properties of Ca2+-ATPase in a characteristic way: V is decreased and the modulatory effect of high ATP concentrations is reduced, in contrast to what occurs by a rise of temperature. The intrinsic fluorescence of the protein is increased, especially in the absence of Ca2+, suggesting that C12E8 modified in particular the E* form (Ca2+-depleted conformation) of the enzyme. Furthermore, stopped-flow data indicate that C12E8 strongly activates the E* to E transition, which may account for the effect of the detergent on ATP modulation during steady-state ATP hydrolysis. It is concluded that C12E8 perturbs ATPase turnover by direct interaction with the enzyme, rather than by an indirect effect exerted via a change in the lipid phase or protein aggregation.

Rotational dynamics of protein and boundary lipid in sarcoplasmic reticulum membrane

We have used spin labels and electron paramagnetic resonance (EPR) to study the correlation between the rotational dynamics of protein and lipid in sarcoplasmic reticulum (SR) membranes. A short-chain maleimide spin label was used to monitor the submillisecond rotational mobility of the Ca-ATPase enzyme (using saturation transfer EPR); a free fatty acid spin label was used to monitor the submicrosecond rotational mobility of the bulk lipid hydrocarbon chains (using conventional EPR); and a fatty acid spin label derivative (long-chain maleimide) attached to the enzyme was used to monitor the mobility of hydrocarbon chains adjacent to the protein (i.e., boundary lipid). In the native SR membranes, the protein was highly mobile (effective correlation time 50 microseconds). The spectra of the hydrocarbon probes both contained at least two components. For the unattached probe, the major component indicated nearly as much mobility as in the absence of protein (effective rotational correlation time 3 ns), while a minor component, corresponding to 25-30% of the total signal, indicated strong immobilization (effective correlation time greater than or equal to 10 ns). For the attached hydrocarbon probe, the major component (approximately 70% of the total) was strongly immobilized, and the mobile component was less mobile than that of the unattached probe. When the lipid-to-protein ratio was reduced 55% by treatment with deoxycholate, protein mobility decreased considerably, suggesting protein aggregation. A concomitant increase was observed in the fraction of immobilized spin labels for both the free and attached hydrocarbon probes. The observed hydrocarbon immobilization probably arises in part from immobilization at the protein-lipid boundary, but protein-protein interactions that trap hydrocarbon chains may also contribute. When protein aggregation was induced by glutaraldehyde crosslinking, submillisecond protein mobility was eliminated, but there was no effect on either hydrocarbon probe. Thus protein aggregation does not necessarily cause hydrocarbon chain immobilization.