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* Biochemistry Section, Bax, a member of the Bcl-2 protein family,
accelerates apoptosis by an unknown mechanism. Bax
has been recently reported to be an integral membrane
protein associated with organelles or bound to organelles by Bcl-2 or a soluble protein found in the cytosol. To explore Bcl-2 family member localization in living cells, the green fluorescent protein (GFP) was fused
to the NH2 termini of Bax, Bcl-2, and Bcl-XL. Confocal
microscopy performed on living Cos-7 kidney epithelial
cells and L929 fibroblasts revealed that GFP-Bcl-2 and
GFP-Bcl-XL had a punctate distribution and colocalized with a mitochondrial marker, whereas GFP-Bax
was found diffusely throughout the cytosol. Photobleaching analysis confirmed that GFP-Bax is a soluble
protein, in contrast to organelle-bound GFP-Bcl-2. The
diffuse localization of GFP-Bax did not change with
coexpression of high levels of Bcl-2 or Bcl-XL. However, upon induction of apoptosis, GFP-Bax moved intracellularly to a punctate distribution that partially
colocalized with mitochondria. Once initiated, this Bax
movement was complete within 30 min, before cellular
shrinkage or nuclear condensation. Removal of a
COOH-terminal hydrophobic domain from GFP-Bax
inhibited redistribution during apoptosis and inhibited
the death-promoting activity of both Bax and GFP-
Bax. These results demonstrate that in cells undergoing
apoptosis, an early, dramatic change occurs in the intracellular localization of Bax, and this redistribution of soluble Bax to organelles appears important for Bax to
promote cell death.
Light Microscopy Facility, Laboratory of Molecular Biology, National
Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892
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