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J. Biol. Chem., Vol. 277, Issue 13, 10973-10981, March 29, 2002
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,
From the Department of Chemistry and Biochemistry, University of
California, Los Angeles, California 90095-1569 and the
§ Laboratorio Andaluz de Biología, Universidad Pablo
de Olavide, 41013 Sevilla, Spain
Coenzyme Q (Q) is an essential
component of the mitochondrial respiratory chain in eukaryotic
cells but also is present in other cellular membranes where it
acts as an antioxidant. Because Q synthesis machinery in
Saccharomyces cerevisiae is located in the mitochondria,
the intracellular distribution of Q indicates the existence of
intracellular Q transport. In this study, the uptake of exogenous
Q6 by yeast and its transport from the plasma membrane to
mitochondria was assessed in both wild-type and in Q-less
coq7 mutants derived from four distinct laboratory yeast strains. Q6 supplementation of medium containing ethanol, a
non-fermentable carbon source, rescued growth in only two of the four
coq7 mutant strains. Following culture in medium containing
dextrose, the added Q6 was detected in the plasma membrane
of each of four coq7 mutants tested. This detection of
Q6 in the plasma membrane was corroborated by measuring
ascorbate stabilization activity, as catalyzed by NADH-ascorbate free
radical reductase, a transmembrane redox activity that provides a
functional assay of plasma membrane Q6. These assays
indicate that each of the four coq7 mutant strains assimilate exogenous Q6 into the plasma membrane.
The two coq7 mutant strains rescued by
Q6 supplementation for growth on ethanol contained
mitochondrial Q6 levels similar to wild type.
However, the content of Q6 in mitochondria from the
non-rescued strains was only 35 and 8%, respectively, of that present
in the corresponding wild-type parental strains. In yeast strains
rescued by exogenous Q6, succinate-cytochrome c
reductase activity was partially restored, whereas non-rescued strains
contained very low levels of activity. There was a strong correlation
between mitochondrial Q6 content, succinate-cytochrome
c reductase activity, and steady state levels of the
cytochrome c1 polypeptide. These studies show
that transport of extracellular Q6 to the mitochondria
operates in yeast but is strain-dependent. When Q
biosynthesis is disrupted in yeast strains with defects in the
intracellular transport of exogenous Q, the bc1
complex is unstable. These results indicate that delivery of exogenous
Q6 to mitochondria is required fore activity and stability
of the bc1 complex in yeast coq mutants.
Recipient of a Fulbright postdoctoral fellowship from the Spanish
Ministry of Education and Science.
¶
To whom correspondence should be addressed: Dept. of Chemistry
and Biochemistry, University of California, 607 Charles E. Young Dr.
East, Los Angeles, CA 90095-1569. Tel.: 310-825-0771; Fax:
310-206-5213; E-mail: cathy@mbi.ucla.edu.
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