Small multidrug resistance protein EmrE phenotypically associates with OmpW, DcrB and YggM for osmotic stress protection by betaine in .

The small multidrug resistance (SMR) protein EmrE resides in the inner membrane and provides resistance against a wide range of antiseptic quaternary cationic compounds (QCCs) for the Gram-negative bacterium . We have reported previously that overexpression of the gene results in the reduction of pH and osmotic tolerance, likely through EmrE-mediated biological QCC-based osmoprotectant efflux, indicating a potential physiological role for EmrE beyond providing drug resistance. EmrE is the most studied member of SMR transporter family; however, it is not known how the substrates translocated by EmrE move across the periplasm and through the outer membrane (OM). We have shown that the OM protein OmpW participates in the EmrE-mediated substrate efflux process and provided a hypothesis for the present study that additional OM and periplasmic proteins participate in the translocation process. To test the hypothesis, we conducted alkaline pH-based growth phenotype screens under overexpression conditions. This screen identified 10 additional genes that appear to contribute to the EmrE-coupled osmoprotectant efflux: , , , , , , , , and . Further screening of these genes using a hyperosmotic growth phenotype assay in the presence and the absence of the osmoprotectant glycine betaine identified and two periplasmic protein genes, and , are mechanistically linked to EmrE.