Protein oligomerization mediated by the transmembrane carboxyl terminal domain of Bcl-XL

Abstract

a b s t r a c tBcl-XL is a pro-survival member of the Bcl-2 family that can be found in the outer mitochondrial membrane and in soluble cytosolic homodimers. Bcl-XL can bind pro-apoptotic members of this family preventing them from activating the execution phase of apoptosis. Bcl-XL has been shown to homodimerize in different ways, although most binding and structural assays have been carried out in the absence of its carboxyl terminal transmembrane domain. We show here that this domain can by itself direct protein oligomerization, which could be related to its previously reported role in mitochondrial morphology alterations and apoptosis inhibition. Structured summary of protein interactions:Vamp2 physically interacts with Vamp2 by blue native page (View interaction) Vamp2 physically interacts with Vamp2 by cross-linking study (View interaction) Bcl-Xl physically interacts with Bcl-Xl by blue native page (View interaction) Bcl-Xl physically interacts with Bcl-Xl by cross-linking study (View interaction) Ó

Ligand Binding and Membrane Insertion Compete with Oligomerization of the BclXL Apoptotic Repressor

“…BclXL_FL dimer is most likely constructed through TM-swapping, such that the TM domain of one monomer occupies the canonical hydrophobic groove within the other monomer and vice versa in an intermolecular trans-fashion, in agreement with our observations that the binding of BH3 ligands to the canonical hydrophobic groove is compromised in the context of full-length BclXL (Table 1). This notion is also consistent with previous studies in which the TM domain was shown to promote homodimerization of full-length BclXL within live cells 32; 38 . In contrast, the formation of BclXL_dTM dimer is most probably driven through inter-monomer swapping of α6–α8 helices such that the canonical hydrophobic grooves within each monomer remain fully exposed to solution and are available for the binding of BH3 ligands without any restriction as reported earlier in the case of BclXL and BclW constructs in which the TM domain has been truncated 39–41 .…”

“…Inside MOM, BclXL oligomer may exert its anti-apoptotic action by virtue of its ability to interfere with Bax and other effectors in the creation of mitochondrial pores so as to prevent the cytosolic release of apoptogenic factors and thereby halt the cell to undergo apoptosis. Notably, our model presented above is consistent with previous studies implicating the role of TM domain in mediating membrane insertion of apoptotic repressors 28; 32; 38 , but contrasts other studies where regions other than the TM domain have been suggested 44; 67 . More importantly, consistent with our model is the observation that truncation of TM domain in both BclXL and Bcl2 repressors renders them cytosolic and impairs their ability to prevent apoptotic cell death 32; 68 .…”

“…This raises the possibility that the TM domain in BclXL may not only associate with the rest of the protein in an intramolecular cis-manner but rather its intermolecular association in a trans-fashion through TM-swapping may be a preferred alternative as suggested by the BclXL_transTM model (Figure 8c). This latter notion is not only supported by cell-based studies on full-length BclXL 32; 38 , but would also provide a physical route for the oligomerization of BclXL into higher-order oligomers reported here for the first time. Importantly, our BclXL_transTM model suggests that homodimerization, with the monomers related by a two-fold axis of symmetry, would lead to further thermodynamic stabilization of TM domains.…”

“…Importantly, this salient observation is further supported by studies conducted within live cells 32; 38 . However, our study also challenges the findings of other investigators.…”

Acidic pH promotes oligomerization and membrane insertion of the BclXL apoptotic repressor

“…Notably, the truncation of the C-terminal TM domain completely abolished the binding of BclXL to bicelles under all pH conditions (data not shown), implying that the TM domain is a requisite for membrane insertion of BclXL. This salient observation is consistent with previous developments implicating the role of TM domain in mediating membrane insertion of apoptotic repressors [66–68], but contrasts other studies where regions other than the TM domain have been suggested [69, 70]. More importantly, the observation that the truncation of TM domain in both BclXL and Bcl2 repressors renders them cytosolic and impairs their ability to prevent apoptotic cell death may be due to their inability to insert into MOM upon apoptotic induction [71, 67].…”