Bcl-x(L) sequesters its C-terminal membrane anchor in soluble, cytosolic homodimers.
Bcl-x(L) is a potent inhibitor of apoptosis. While Bcl-x(L) can be bound to mitochondria, a substantial fraction, depending on the cell type or tissue, is found in the cytosol of healthy cells. Gel filtration and crosslinking experiments reveal that, unlike monomeric Bax, Bcl-x(L) migrates in a complex of approximately 50 kDa in the cytosol. Co-immunoprecipitation experiments indicate that Bcl-x(L) in the cytosol forms homodimers. The C-terminal hydrophobic tails of two Bcl-x(L) molecules are involved in homodimer formation, and analysis of mutants demonstrates that the C-terminal lysine residue and the G138 residue lining the BH3-binding pocket are required for homodimerization. The flexible loop preceding the C-terminal tail in Bcl-x(L) is longer than that of several monomeric Bcl-2 family members and is a requisite for the homodimer formation. Bad binding to Bcl-x(L) dissociates the homodimers and triggers Bcl-x(L) binding to mitochondrial membranes. The C-terminal tail of Bcl-x(L) is also required to mediate Bcl-x(L)/Bax heterodimer formation. Both mitochondrial import and antiapoptotic activity of different Bcl-x(L) mutants correlate with their ability to form homodimers.
Jeong
Seon-Yong
SY
Biochemistry Section, Surgical Neurology Branch, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892-1414, USA.
Gaume
Brigitte
B
Lee
Yang-Ja
YJ
Hsu
Yi-Te
YT
Ryu
Seung-Wook
SW
Yoon
Soo-Han
SH
Youle
Richard J
RJ
D000595:Amino Acid Sequence; D000818:Animals; D017209:Apoptosis / Q000502:physiology; D001665:Binding Sites; D002352:Carrier Proteins / Q000378:metabolism; D002460:Cell Line; D003593:Cytoplasm / Q000378:metabolism; D019281:Dimerization; D006801:Humans; D008928:Mitochondria / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D010446:Peptide Fragments / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D051028:bcl-2-Associated X Protein; D051025:bcl-Associated Death Protein; D051020:bcl-X Protein
C494819:BAD protein, human; C494826:BAX protein, human; C494811:BCL2L1 protein, human; C474007:Bax protein (53-86); D002352:Carrier Proteins; D010446:Peptide Fragments; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D051028:bcl-2-Associated X Protein; D051025:bcl-Associated Death Protein; D051020:bcl-X Protein