Exploring the role of a unique carboxyl residue in EmrE by mass spectrometry
ABSTRACT EmrE is a small multidrug transporter in Escherichia coli that extrudes various positively charged drugs across the plasma membrane in exchange with protons, thereby rendering cells resistant to these compounds. Biochemical experiments indicate that the basic functional unit of EmrE is a dimer where the common binding site for protons and substrate is formed by the interaction of an essential charged residue (Glu-14) from both EmrE monomers. Carbodiimide modification of EmrE has been studied using functional assays, and the evidence suggests that Glu-14 is the target of the reaction. Here we exploited electrospray ionization mass spectrometry to directly monitor the reaction with each monomer rather than following inactivation of the functional unit. A cyanogen bromide peptide containing Glu-14 allows the extent of modification by the carboxyl-specific modification reagent diisopropylcarbodiimide (DiPC) to be monitored and reveals that peptide 2NPYIYLGGAILAEVIGTTLM(21) is approximately 80% modified in a time-dependent fashion, indicating that each Glu-14 residue in the oligomer is accessible to DiPC. Furthermore, preincubation with tetraphenylphosphonium reduces the reaction of Glu-14 with DiPC by up to 80%. Taken together with other biochemical data, the findings support a "time sharing" mechanism in which both Glu-14 residues in a dimer are involved in tetraphenylphosphonium and H(+) binding.
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ABSTRACT: EmrE is a small (110 residues) SMR transporter from Escherichia coli that extrudes positively charged aromatic drugs in exchange for two protons, thus rendering bacteria resistant to a variety of toxic compounds. Due to its size, stability and retention of its function upon solubilization in detergent, EmrE provides a unique experimental paradigm for the biochemical and biophysical studies of membrane based ion-coupled transporters. In addition, EmrE has been in center stage in the past two years because it provides also a paradigm for the study of the evolution of membrane proteins. Controversy around this topic is still going on and some novel concepts are surfacing that may contribute to our understanding of evolution of topology of membrane proteins. Furthermore, based on the findings that the cell multidrug transporters interact functionally we introduce the concept of a cell Resistosome.Biochimica et Biophysica Acta 05/2009; DOI:10.1016/j.bbapap.2008.12.018