Interactions of free and immobilized myelin basic protein with anionic detergents

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Abstract
The interaction of free and immobilized bovine myelin basic protein (MBP) with sodium deoxycholate (DOC) and sodium dodecyl sulfate (NaDodSO4) was studied under a variety of conditions. Free MBP formed insoluble complexes with both detergents. Analysis of the insoluble complexes revealed that the molar ratio of detergent/MBP in the precipitate increased in a systematic fashion with increasing detergent concentration until the complex became soluble. At pH 4.8, equilibrium dialysis studies indicated that .apprx. 15 mol of NaDodSO4 could bind to the protein without precipitation occurring. Regardless of the surfactant, minimum protein solubility occurred when the net charge on the protein-detergent complex was between +18 and -9. Complete equilibrium binding isotherms of both detergents to the protein were obtained by using MBP immobilized on agarose. The bulk of the binding of both detergents was highly cooperative and occurred at or above the critical micelle concentration. At I = 0.1, saturation levels of 2.09 .+-. 0.15 g of NaDodSO4/g of protein and 1.03 .+-. 0.40 g of DOC/g of protein were obtained. Below pH 7.0 the NaDodSO4 binding isotherms revealed an additional cooperative transition corresponding to the binding of 15-20 mol of NaDodSO4/mol of protein. Affinity chromatography studies indicated that in the presence of NaDodSO4 (but not in its absence), [125I]MBP interacted with agarose-immobilized histone, lysozyme and MBP but did not interact with ovalbumin-agarose. The data support a model in which the detergent cross-links and causes precipitation of MBP-anionic detergent complexes. Cross-linking may occur through hydrophobic interaction between detergents electrostatically bound to different MBP molecules.