A Survey for Small Multidrug Resistance Protein Multimerization in the Presence of Ligand Using Sds-Page Analysis

Abstract

EmrE and SugE are members of the small multidrug resistance protein family that can efflux quaternary cation compounds (QCC) via proton motive force within the Escherichia coli plasma membrane. Members of this integral membrane protein family are characterized by their short (∼100-140 amino acids) four transmembrane (TM) alpha- helix conformation and highly conserved glutamate residue within the active site. EmrE protein can confer broad multidrug resistance to the host strain unlike SugE protein, which demonstrates limited multidrug resistance. The exact multimeric state or states of both proteins during transport and ligand binding is not well understood and often yield conflicting results that are specific to the conditions of study. To explore SMR multimerization as influenced by QCC ligands, organic solvent extracted EmrE and SugE protein from E. coli membranes were characterized in the detergents, sodium dodecyl sulfate (SDS) and dodecyl maltoside (DDM) at varying protein concentrations. SMR proteins solubilized in both detergents demonstrated a predominately monomeric state but upon increasing particular QCC ligand concentrations resulted in multimer formation or enhancement using SDS- tricine polyacrylamide gel electrophoresis (PAGE). The results from this PAGE based assay demonstrate that: i) SMR multimers are induced by particular ligands that may relate to ligand shape and ii) only EmrE multimerization is induced by particular ligands, whereas SugE appears to be insensitive to drug enhanced oligomerization. Therefore, SMR multimer variability may be dependent upon the nature of the transported substrate and SMR subclass; only EmrE can alter its subunit composition in response to particular QCC substrates. This PAGE based assay provides the framework to explore the influence of diverse QCC substrates for its affects on SMR multimerization.