Mrp systems of Gram-positive bacteria: Properties of the monovalent cation/proton antiport.

摘要

The Mrp family of bacterial monovalent cation/proton antiporters is widely distributed and physiologically important. These antiporters are unusual in their requirement for 6-7 hydrophobic gene products, some of which resemble respiratory chain Complex I subunits. Other monovalent cation/proton antiporters are single gene products that catalyze secondary antiport only. It has been proposed that the complexity of Mrp reflects a capacity to catalyze primary antiport that achieves net generation, rather than consumption, of the transmembrane potential during antiport. Lack of an efficacious assay has impeded characterization of the secondary antiport capacity of Mrp. Here, an improved assay for secondary monovalent cation/proton antiport was used to study Mrp antiporter systems from alkaliphilic Bacillus pseudofirmus OF4, neutrophilic Bacillus subtilis, and the pathogen and facultative alkaliphile Staphylococcus aureus in transformants of antiporter-deficient Escherichia coli EP432. These systems catalyze Na+ and Li+/H+ antiport, with an optimal pH in the alkaline range, and apparent Km values for Na + within the range of the lowest reported for bacterial Na +/H+ antiporters. The antiport of Mrp systems from both B. pseudofirmus OF4 and from S. aureus are electrogenic. S. aureus has a second Mrp system that supports alkali-resistance in E. coli EP432, but does not confer monovalent cation/proton antiport activity with any of a typical panel of cation substrates; it is hypothesized to function with organic co-substrates. During the current studies, Mrp-dependent enhancement of the capacity for generation of a transmembrane potential was observed in an E. coli host. Analogous observations by others had been used to bolster the argument for a primary Mrp energization mode. The current studies show that the Mrp-dependent increase in transmembrane potential generation was accompanied by increased expression and/or activity of respiratory chain components of the host bacteria. This is hypothesized to be a compensatory cellular response to the energy-depleting effects of Mrp-dependent consumption of transmembrane potential during secondary antiport. The results demonstrate that the Mrp systems studied here are robust secondary electrogenic monovalent cation/proton antiporters and provide a model for explaining how secondary electrogenic antiporters such as Mrp can enhance the capacity of the cells in which they function to generate a transmembrane potential.