MEMBRANE-PROMOTED UNFOLDING OF ACETYLCHOLINESTERASE - A POSSIBLE MECHANISM FOR INSERTION INTO THE LIPID BILAYER

Acetylcholinesterase from Torpedo californica partially unfolds to a s tate with the physicochemical characteristics of a ''molten globule'' upon mild thermal denaturation or upon chemical modification of a sing le nonconserved buried cysteine residue, Cys(231). The protein in this state binds tightly to liposomes. It is here shown that the rate of u nfolding Is greatly enhanced in the presence of unilamellar vesicles o f dimyristoylphosphatidylcholine, with concomitant incorporation of th e protein into the lipid bilayer. Arrhenius plots reveal that in the p resence of the liposomes the energy barrier for transition from the na tive to the molten globule state is lowered from 145 to 47 kcal/mol, C hemical modification of Cys(231) by mercuric chloride produces initial ly a quasi-native state of Torpedo acetylcholinesterase which, at room temperature, undergoes spontaneous transition to a molten globule sta te with a half-life of 1-2 hr, This permitted temporal resolution of i nteraction of the quasi-native state with the membrane from the transi tion of the membrane-bound protein to the molten globule state. The da ta presented here suggest that either the native enzyme, or a quasi-na tive state with which it is in equilibrium. interacts with the liposom e, which then promotes a fast transition to the membrane-bound molten globule state by lowering thr energy barrier for the transition, These findings raise the possibility that the membrane itself, hy lowering the energy barrier for transition to a partially unfolded state, mag p lay an active posttranslational role In Insertion and translocation of proteins in situ.