The Lysine 299 Residue Endows the Multisubunit Mrp1 Antiporter with Dominant Roles in Na + Resistance and pH Homeostasis in Corynebacterium glutamicum

Corynebacterium glutamicum is generally regarded as a moderately salt- and alkali-tolerant industrial organism. However, relatively little is known about the molecular mechanisms underlying these specific adaptations. Here, we found that the Mrp1 antiporter played crucial roles in conferring both environmental Na+ resistance and alkali tolerance whereas the Mrp2 antiporter was necessary in coping with high-KCl stress at alkaline pH. Furthermore, the Δmrp1 Δmrp2 double mutant showed the most-severe growth retardation and failed to grow under high-salt or alkaline conditions. Consistent with growth properties, the Na+ /H+ antiporters ofC. glutamicum were differentially expressed in response to specific salt or alkaline stress, and an alkaline stimulus particularly induced transcript levels of the Mrp-type antiporters. When the major Mrp1 antiporter was overwhelmed,C. glutamicum might employ alternative coordinate strategies to regulate antiport activities. Site-directed mutagenesis demonstrated that several conserved residues were required for optimal Na+ resistance, such as Mrp1A K299 , Mrp1C I76 , Mrp1A H230 , and Mrp1D E136 . Moreover, the chromosomal replacement of lysine 299 in the Mrp1A subunit resulted in a higher intracellular Na+ level and a more alkaline intracellular pH value, thereby causing a remarkable growth attenuation. Homology modeling of the Mrp1 subcomplex suggested two possible ion translocation pathways, and lysine 299 might exert its effect by affecting the stability and flexibility of the cytoplasm-facing channel in the Mrp1A subunit. Overall, these findings will provide new clues to the understanding of salt-alkali adaptation duringC. glutamicum stress acclimatization.IMPORTANCE The capacity to adapt to harsh environments is crucial for bacterial survival and product yields, including industrially usefulCorynebacterium glutamicum . AlthoughC. glutamicum exhibits a marked resistance to salt-alkaline stress, the possible mechanism for these adaptations is still unclear. Here, we present the physiological functions and expression patterns ofC. glutamicum putative Na+ /H+ antiporters and conserved residues of Mrp1 subunits, which respond to different salt and alkaline stresses. We found that the Mrp-type antiporters, particularly the Mrp1 antiporter, played a predominant role in maintaining intracellular nontoxic Na+ levels and alkaline pH homeostasis. Loss of the major Mrp1 antiporter had a profound effect on gene expression of other antiporters under salt or alkaline conditions. The lysine 299 residue may play its essential roles in conferring salt and alkaline tolerance by affecting the ion translocation channel of the Mrp1A subunit. These findings will contribute to a better understanding of Na+ /H+ antiporters in sodium antiport and pH regulation.