NqrM (DUF539) protein is required for maturation of bacterial Na+- translocating NADH: Quinone oxidoreductase

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Abstract

Na+-translocating NADH:quinone oxidoreductase (Na+-NQR) catalyzes electron transfer from NADH to ubiquinone in the bacterial respiratory chain, coupled with Na+ translocation across the membrane. Na+-NQR maturation involves covalent attachment of flavin mononucleotide (FMN) residues, catalyzed by flavin transferase encoded by the nqr-associated apbE gene. Analysis of complete bacterial genomes has revealed another putative gene (duf539, here renamed nqrM) that usually follows the apbE gene and is present only in Na+-NQR-containing bacteria. Expression of the Vibrio harveyi nqr operon alone or with the associated apbE gene in Escherichia coli, which lacks its own Na+-NQR, resulted in an enzyme incapable of Na+-dependent NADH or reduced nicotinamide hypoxanthine dinucleotide (dNADH) oxidation. However, fully functional Na+-NQR was restored when these genes were coexpressed with the V. harveyi nqrM gene. Furthermore, nqrM lesions in Klebsiella pneumoniae and V. harveyi prevented production of functional Na+-NQR, which could be recovered by an nqrM-containing plasmid. The Na+-NQR complex isolated from the nqrM-deficient strain of V. harveyi lacks several subunits, indicating that nqrM is necessary for Na+-NQR assembly. The protein product of the nqrM gene, NqrM, contains a single putative transmembrane α-helix and four conserved Cys residues. Mutating one of these residues (Cys33 in V. harveyi NqrM) to Ser completely prevented Na+- NQR maturation, whereas mutating any other Cys residue only decreased the yield of the mature protein. These findings identify NqrM as the second specific maturation factor of Na+-NQR in proteobacteria, which is presumably involved in the delivery of Fe to form the (Cys)4[Fe] center between subunits NqrD and NqrE.

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Kostyrko, V. A., Bertsova, Y. V., Serebryakova, M. V., Baykov, A. A., & Bogachev, A. V. (2016). NqrM (DUF539) protein is required for maturation of bacterial Na+- translocating NADH: Quinone oxidoreductase. Journal of Bacteriology, 198(4), 655–663. https://doi.org/10.1128/JB.00757-15

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