Phase behavior of membranes reconstituted from dipentadecanoylphosphatidylcholine and the Mg2+-dependent, Ca2+-stimulated adenosinetriphosphatase of sarcoplasmic reticulum: evidence for a disrupted lipid domain surrounding protein

A new method was used for reconstituting active sodium deoxycholate solubilized Ca2+-ATPase of rabbit skeletal muscle sarcoplasmic reticulum. Removal of the detergent by dialysis at the pretransition temperature of the pure lipid (22 degrees C) favored the formation of sheet-like structures with a lipid and protein content close to that of the detergent-solubilized sample. Freeze-fracture electron micrographs revealed the Ca2+-ATPase to be organized in rows corresponding to the typical banded pattern seen in low-temperature freeze-fracture micrographs of pure lipid bilayers. Incubation of the sheetlike structures at a temperature (38 degrees C) above the pure lipid main phase transition (33.5 degrees C) caused closure of the sheets into vesicles displaying homogeneous intramembranous particle distributions, at least for membranes containing less than 150 lipids per Ca2+-ATPase. However, in membranes of higher lipid content, free lipid patches were seen both above and below the lipid phase transition. By use of high-sensitivity differential scanning calorimetry, three classes of excess heat capacity peaks were observed in the vesiculated samples. A broadened "free lipid" peak occurred for samples containing between 550 and 200 lipids per protein (Tm = 33.5 degrees C, as for the order-disorder transition in pure lipid vesicles). Between 200 and 150 lipids per Ca2+-ATPase, a broad shoulder became apparent in the range of 29-32 degrees C. Below 150 lipids per Ca2+-ATPase, a peak at 26-28 degrees C became increasingly prominent with lower lipid content. At a lipid to protein ratio of about 30, no peaks in heat capacity were observed. The temperature dependence of diphenylhexatriene fluorescence anisotropy revealed a similar pattern of membrane phase behavior, except that a phase transition was detected at 33.5 degrees C in all membranes studied. On the basis of these observations, we propose that the Ca2+-ATPase is surrounded by a "lipid annulus" of motionally inhibited lipid molecules that do not contribute to a calorimetrically detectable phase transition. Beyond the annulus, "secondary domains" of disrupted lipid packing account for the peak at 26-28 degrees C and the 29-32 degrees C shoulders. At high lipid to protein ratios, the secondary domains coexist with protein-free, lipid-bilayer patches, which account for the peak at 33.5 degrees C.