A mechanism to alter reversibly the oligomeric state of a membrane-bound protein demonstrated with Escherichia coli EIImtl in solution

This paper reports that the aggregation state of a membrane protein can be changed reversibly without the use of chaotropic agents or denaturants by altering the attractive interactions between micelles of polyethylene glycol-based detergents, This has been documented using mannitol permease of Escherichia coli (EIImtl), a protein whose activity is dependent on the dimerization of its membrane-embedded domains, We show that the driving force for the hydrophobic interactions responsible for the dimerization can be decreased by bringing the protein into a less polar environment, This can be done simply and reversibly by increasing the micelle cluster size of the solubilizing detergent since the micropolarity in the micelle decreases upon clustering and is directly related to the cluster size. The micelle cluster size was varied at a fixed temperature by adding sodium phosphate or a second detergent with a distinct clustering behavior, and the changes were quantified by quasi elastic light scattering and by determining the cloud point or demixing temperature (T-d) of the detergent, Maximal EIImtl activity was found when no micelle clustering occurred, but the activity gradually decreased down to 5% of the maximal activity with increasing cluster size, The inactivation was found to be completely reversible, The kinetics of heterodimer formation were also significantly affected by changes in the micelle cluster size as expected, Increasing the cluster size resulted in faster formation of functional heterodimers by increasing the rate of homodimer dissociation. This phenomenon should be generally applicable to controlling the oligomeric state of membrane-bound proteins or even water-soluble proteins if their subunit association is dominated by hydrophobic forces.