Bcl-2 complexed with Beclin-1 maintains full anti-apoptotic function.
The binding of Bcl-2 to Beclin-1 reduces Beclin-1's capacity to induce autophagy. Here, we have tested whether the interaction is reciprocated by loss of Bcl-2's anti-apoptotic function. We targeted Bcl-2 to mitochondria or endoplasmic reticulum (ER) and induced apoptosis using several apoptotic stimuli that initiate ER and/or mitochondrial signaling pathways (UV radiation, TNF and cycloheximide, staurosporine, thapsigargin and tunicamycin). When Beclin-1 and Bcl-2 were expressed together in HeLa cells, Beclin-1 (but not Beclin-1 lacking the Bcl-2-binding domain) followed Bcl-2 to the appropriate organelle with complete or near-complete overlap (comprising 60 and 30% of cells, respectively). The interaction between Beclin-1 and Bcl-2 was verified by immunoprecipitation, and a membrane-proximate localization of Beclin-1 was shown by immunoelectron microscopy. Apoptosis was followed by measuring changes in nuclear morphology, caspase-3 activity, poly-ADP-ribose polymerase cleavage or punctation of mRFP-Bax on mitochondria. Binding of Beclin-1 to Bcl-2 did not modify apoptosis irrespective of Bcl-2 concentration, location or apoptotic stimulus. A similar result was obtained in Atg5-/- cells that are autophagy-deficient, arguing against compensation for the loss of protection by Bcl-2 by autophagy-mediated survival induced by Beclin-1. Hence, although Beclin-1 contains a BH3-only motif typical of pro-apoptotic proteins, it is a negligible modulator of Bcl-2's anti-apoptotic function.
Ciechomska
I A
IA
Department of Biochemistry, University of Cambridge, Cambridge, UK.
Goemans
G C
GC
Skepper
J N
JN
Tolkovsky
A M
AM
D000818:Animals; D017209:Apoptosis; D051017:Apoptosis Regulatory Proteins / Q000235:genetics / Q000378:metabolism*; D000071186:Beclin-1; D002478:Cells, Cultured; D005786:Gene Expression Regulation; D006801:Humans; D042541:Intracellular Space / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D011485:Protein Binding; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000235:genetics / Q000378:metabolism*
D051017:Apoptosis Regulatory Proteins; C491997:BECN1 protein, human; D000071186:Beclin-1; D008565:Membrane Proteins; D019253:Proto-Oncogene Proteins c-bcl-2