The proton-pumping NADH:ubiquinone oxidoreductase, respiratory complex I,
couples the electron transfer from NADH to ubiquinone with the translocation
of protons across the membrane. In Escherichia coli the complex is made up of
13 different subunits encoded by the so-called nuo-genes. Mutants, in which
each of the nuo-genes was individually disrupted by the insertion of a
resistance cartridge were unable to assemble a functional complex I. Each
disruption resulted in the loss of complex I-mediated activity and the failure
to extract a structurally intact complex. Thus, all nuo-genes are required
either for the assembly or the stability of a functional E. coli complex I.
The three subunits comprising the soluble NADH dehydrogenase fragment of the
complex were detected in the cytoplasm of several nuo-mutants as one distinct
band after BN-PAGE. It is discussed that the fully assembled NADH
dehydrogenase fragment represents an assembly intermediate of the E. coli
complex I. A partially assembled complex I bound to the membrane was detected
in the nuoK and nuoL mutants, respectively. Overproduction of the ΔNuoL
variant resulted in the accumulation of two populations of a partially
assembled complex in the cytoplasmic membranes. Both populations are devoid of
NuoL. One population is enzymatically active, while the other is not. The
inactive population is missing cluster N2 and is tightly associated with the
inducible lysine decarboxylase. This article is part of a Special Issue
entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes
