Characterization of the Membrane Domain Subunit NuoK (ND4L) of the NADH-Quinone Oxidoreductase from Escherichia coli

• Published in: Biochemistry (Easton) 2005-07, Vol.44 (27), p.9545-9554
• Main Authors: Kao, Mou-Chieh, Nakamaru-Ogiso, Eiko, Matsuno-Yagi, Akemi, Yagi, Takao
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Conserved Sequence; Electron Transport Complex I - chemistry; Escherichia coli Proteins - chemistry; Escherichia coli Proteins - genetics; Glutamic Acid - genetics; Membrane Proteins - chemistry; Membrane Proteins - deficiency; Membrane Proteins - genetics; Molecular Sequence Data; Mutagenesis, Site-Directed; NADH Dehydrogenase - chemistry; NADH Dehydrogenase - deficiency; NADH Dehydrogenase - genetics; Point Mutation; Potentiometry; Protein Processing, Post-Translational - genetics; Protein Structure, Tertiary - genetics; Protein Subunits - chemistry; Protein Subunits - deficiency; Protein Subunits - genetics; Quinone Reductases - chemistry; Quinone Reductases - deficiency; Quinone Reductases - genetics; Sequence Analysis, Protein
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi050708w

The ND4L subunit is the smallest mitochondrial DNA-encoded subunit of the proton-translocating NADH-quinone oxidoreductase (complex I). In an attempt to study the functional and structural roles of the NuoK subunit (the Escherichia coli homologue of ND4L) of the bacterial NADH-quinone oxidoreductase (NDH-1), we have performed a series of site-specific mutations on the nuoK gene of the NDH-1 operon by using the homologous recombination technique. The amino acid residues we targeted included two highly conserved glutamic acids that are presumably located in the middle of the membrane and several arginine residues that are predicted to be on the cytosolic side. All point mutants examined had fully assembled NDH-1 as detected by blue-native gel electrophoresis and immunostaining. Mutations of nearly perfectly conserved Glu-36 lead to almost null activities of coupled electron transfer with a concomitant loss of generation of electrochemical gradient. A significant diminution of the coupled activities was also observed with mutations of another highly conserved residue, Glu-72. These results may suggest that both membrane-embedded acidic residues are important for the coupling mechanism of NDH-1. Furthermore, a severe impairment of the coupled activities occurred when two vicinal arginine residues on a cytosolic loop were simultaneously mutated. Possible roles of these arginine residues and other conserved residues in the NuoK subunit for NDH-1 function were discussed.