Micelle-Catalyzed Domain Swapping in the Cytoplasmic Domain of E. coli Rhomboid Protease

Abstract

Domain swapping is a mechanism of protein oligomerization whereby subunits exchange identical structural elements with each other. This type of interaction is important in a number of biologically important processes, including the regulation of enzyme activity, the modulation of molecular recognition and in the development of some protein deposition diseases. Domain swapping can be promoted by exposure to chemical denaturants, high temperatures, or redox reagents, although these factors can deviate greatly from conditions encountered in vivo. Moreover, in some cases these conditions can alter the monomer-oligomer equilibrium or even promote the formation of alternate domain-swapped oligomers. In contrast, we have found that it is possible for detergent micelles to be used as a catalyst in the domain-swapping interaction involving the N-terminal cytoplasmic domain of E. coli GlpG rhomboid protease. Our results show that hexadecylphosphocholine micelles can catalyze domain swapping of NGlpG by lowering the Gibbs free energy of the kinetic barrier by 12 kcal/mol, while preserving the equilibrium populations of monomer and dimer. Micelle charge and size were found to be important for this catalysis, which involved the formation of a partially unfolded micelle-bound intermediate containing significant secondary structure. Overall, the results from this work reveal how detergent micelles can affect the energy landscape of domain swapping for NGlpG, and provide insight into the potential ability of local irregularities in lipid membrane environments to play a role in domain swapping in vivo.