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Published online 22 February 2005. doi:10.1083/jcb.200408169
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 168, Number 5, 723-734
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Article

 Bcl-XL specifically activates Bak to induce swelling and restructuring of the endoplasmic reticulum

Martina Klee and Felipe X. Pimentel-Muiños

Centro de Investigación del Cáncer, Universidad de Salamanca-CSIC, Salamanca, 37007 Spain

Correspondence to F.X. Pimentel-Muiños: fxp{at}usal.es

Bcl-2 family members Bak and Bax constitute a mitochondrial gateway for multiple death pathways. Both proteins are also present in the endoplasmic reticulum where they control apoptosis through the regulation of calcium levels. We show here that reticular Bak has the additional capacity of modulating the structure of this organelle. Coexpression of Bak and Bcl-XL provokes extensive swelling and vacuolization of reticular cisternae. A Bak version lacking the BH3 domain suffices to induce this phenotype, and reticular targeting of this mutant retains the activity. Expression of upstream BH3-only activators in similar conditions recapitulates ER swelling and vacuolization if ryanodine receptor calcium channel activity is inhibited. Experiments with Bak and Bax-deficient mouse embryonic fibroblasts show that endogenous Bak mediates the effect, whereas Bax is mainly irrelevant. These results reveal a previously unidentified role of Bak in regulating reticular conformation. Because this activity is absent in Bax, it constitutes one of the first examples of functional divergence between the two multidomain homologues.

Abbreviations used in this paper: BH, Bcl-2 homology; erRFP, ER-targeted RFP; FRET, fluorescence resonance energy transfer; MEFs, mouse embryonic fibroblasts; PARP, poly(ADP-ribose) polymerase; RyR, ryanodine receptor; tBid, truncated version of Bid.


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