The concept of boundary lipid as it pertains to the calcium-pump protein of sarcoplasmic reticulum

Role of lipids in sarcoplasmic reticulum: a higher lipid content is required to sustain phosphoenzyme decomposition than phosphoenzyme formation

Enzyme preparations with variable phospholipid contents were obtained by removing lipids from sarcoplasmic reticulum with deoxycholate. Preparations containing from 90 to 37 phospholipids per enzyme showed normal values of both Ca2+-ATPase activity and steady-state phosphoenzyme levels. Fractions containing 37 to 23 phospholipids per enzyme had a reduced ATPase activity but normal phosphoenzyme levels, showing that in this range of lipid content the ATPase reaction is inhibited in a reaction step subsequent to phosphoenzyme formation but prior to phosphoenzyme decomposition. Delipidation below 23 lipids per enzyme caused a marked reduction of the amount of phosphoenzyme formed, so that although both reactions require lipids, fewer lipids are required for phosphoenzyme formation than for decomposition. The effect of lipid removal could be completely reversed by readdition of lipids to fractions containing more than 11 lipids per enzyme. It is proposed that phosphoenzyme formation requires full occupancy of a boundary domain of 23 lipids per enzyme, and that the selective inhibition of phosphoenzyme decomposition at higher lipid contents is caused by a decrease in the rotational mobility of the enzyme.

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.

Characterization of sarcoplasmic reticulum adenosinetriphosphatase purified by selective column adsorption

Preparations of sarcoplasmic reticulum ATPase made by conventional procedures, with over 85% of the protein consisting of one band in sodium dodecyl sulfate gel electrophoresis, were solubilized in Triton X-100 and separated on an Affi-Gel blue column. All the ATPase activity was eluted in a single fraction containing about 60% of the applied protein. This purified fraction required combination with about 1 mol of fluoresceinyl 5-isothiocyanate (FITC) for inactivation, whereas the original preparation was inactivated by reaction with about 0.6 mol of FITC/mol. The inactive protein retained on the column had an amino acid composition like that of the active protein. The separation on the Affi-Gel blue column provides a convenient procedure for preparation of more active ATPase. The rate of inactivation of the ATPase solubilized in detergent-containing solutions was measured at different protein concentrations. The t1/2 for inactivation was proportional to the square root of the protein concentration. Results are consistent with inactivation proceeding through a small fraction of monomeric ATPase present.