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Mol. Cell. Biol., Feb 1995, 964-974, Vol 15, No. 2
G Hofhaus and G Attardi
The mitochondrial NADH dehydrogenase (complex I) in mammalian cells is a
multimeric enzyme consisting of approximately 40 subunits, 7 of which are
encoded in mitochondrial DNA (mtDNA). Very little is known about the
function of these mtDNA-encoded subunits. In this paper, we describe the
efficient isolation from a human cell line of mutants affected in any of
these subunits. In the course of analysis of eight mutants of the human
cell line VA2B selected for their resistance to high concentrations of the
complex I inhibitor rotenone, seven were found to be respiration deficient,
and among these, six exhibited a specific defect of complex I. Transfer of
mitochondria from these six mutants into human mtDNA-less cells revealed,
surprisingly, in all cases a cotransfer of the complex I defect but not of
the rotenone resistance. This result indicated that the rotenone resistance
resulted from a nuclear mutation, while the respiration defect was produced
by an mtDNA mutation. A detailed molecular analysis of the six complex I-
deficient mutants revealed that two of them exhibited a frameshift mutation
in the ND4 gene, in homoplasmic or in heteroplasmic form, resulting in the
complete or partial loss, respectively, of the ND4 subunit; two other
mutants exhibited a frameshift mutation in the ND5 gene, in
near-homoplasmic or heteroplasmic form, resulting in the ND5 subunit being
undetectable or strongly decreased, respectively. It was previously
reported (G. Hofhaus and G. Attardi, EMBO J. 12:3043-3048, 1993) that the
mutant completely lacking the ND4 subunit exhibited a total loss of NADH:Q1
oxidoreductase activity and a lack of assembly of the mtDNA-encoded
subunits of complex I. By contrast, in the mutant characterized in this
study in which the ND5 subunit was not detectable and which was nearly
totally deficient in complex I activity, the capacity to assemble the
mtDNA-encoded subunits of the enzyme was preserved, although with a
decreased efficiency or a reduced stability of the assembled complex. The
two remaining complex I-deficient mutants exhibited a normal rate of
synthesis and assembly of the mtDNA-encoded subunits of the enzyme, and the
mtDNA mutation(s) responsible for their NADH dehydrogenase defect remains
to be identified. The selection scheme used in this work has proven to be
very valuable for the isolation of mutants from the VA2B cell line which
are affected in different mtDNA-encoded subunits of complex I and may be
applicable to other cell lines.
Copyright © 1995, American Society for Microbiology
Efficient selection and characterization of mutants of a human cell line which are defective in mitochondrial DNA-encoded subunits of respiratory NADH dehydrogenase [published erratum appears in Mol Cell Biol 1995 Jun;15(6):3461]
Division of Biology, California Institute of Technology, Pasadena 91125.
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