Wild-type and mutant forms of bacteriorhodopsin (sbR) from Halobacterium salinarium, produced by Escherichia coli overexpression of a synthetic gene, were reversibly unfolded in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 3-[(3-cholamidopropyl)dimethylamino]-2-hydroxyl-1-propane (CHAPSO), and sodium dodecyl sulfate (SDS) mixed micelles. To study the effect on protein stability by substitutions on the hydrophobic surface with polar residues, the unfolding behavior of a G113Q, G116Q mutant [sbR(Q2)] was compared to the wild-type sbR [sbR(WT)]. sbR(Q2) was more sensitive to SDS- induced unfolding than sbR(WT) under equilibrium conditions, and kinetic experiments showed that sbR(Q2) was more sensitive to acid-induced denaturation and thermal unfolding than sbR(WT). Since the mutations in sbR(Q2) were on the detergent-embedded hydrophobic surface of sbR, protein destabilization by these mutations supports the concept that the membrane- embedded segments are important for the stability of sbR. Our experiments provide the basis for studying he thermodynamic stability of sbR by evaluating reversible folding and unfolding conditions in DMPC/CHAPSO/SDS mixed micelles.
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