Bcl-xL sequesters its C-terminal membrane anchor in soluble, cytosolic homodimers

Abstract

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.

BCL-xL: time-dependent dissociation between modulation of apoptosis and invasiveness in human malignant glioma cells

“…3 Moreover, a Raf-1-binding site within the BH4 domain of BCL-2 might integrate pathways of resistance towards apoptosis with other cellular-signaling events. [18][19][20] It was therefore tempting to speculate that overexpression of BCL-x L might result in a translocation of BCL-x L to intracellular structures other than the mitochondria, for example the cytoskeleton or the inner cell membrane.…”

“…2 However, a substantial fraction of antiapoptotic BCL-x L has been found in a 50 kDa cytosolic homodimer complex in nontumor cells exogenously expressing BCL-x L , addressing the question of additional cytosolic effects. 3 Other BCL-2 family proteins locate physiologically to intracellular membranes. 4 BCL-2 family proteins have mainly been studied as positive or negative regulators of apoptosis.…”

Death pathways associated with photodynamic therapy

“…In contrast, Oleinick's group had found that Bcl-x L was as sensitive as Bcl-2 to photodamage in adhering cells [14]. This difference was eventually traced to a unique property of many suspension cell cultures, i.e., localization of Bcl-x L in the cytosol [15]. Fractionation studies revealed that this was also true for the mouse leukemia L1210 cell line, but not for adhering MCF-10A cells (Fig.…”

Bax activation and mitochondrial insertion during apoptosis

“…Jeong et al [50] demonstrated that these dimers involved the binding of the C-terminal α helix of one protein to the hydrophobic groove of the second one. Since the Bax structure is very similar to that of Bcl-xl under native conditions, Jeong et al [50] reported a possible hetero-dimerisation between Bax and Bcl-xl involving the replacement of Bax Hα9 by Bcl-xl CT in the hydrophobic cleft. Yet, this interaction was only seen in the presence of detergents or with a C-truncated form of Bax, suggesting that a first modification in the structure of Bax is necessary to unlock the protein.…”