A Structural Model for Bax∆2-Mediated Activation of Caspase 8-Dependent Apoptosis

Abstract

Bax∆2 is a pro-apoptotic anti-tumor protein in the Bax family. While most of the Bax family causes cell death by targeting mitochondria, Bax∆2 forms cytosolic aggregates and activates caspase 8-dependent cell death. We previously showed that the Bax∆2 helix α9 is critical for caspase 8 recruitment. However, the interaction between these two proteins at the structural level is unknown. In this in silico study, we performed molecular dynamics (MD) simulations and protein–protein docking on Bax∆2 variants. The results suggest that the Bax∆2 variants have different stable states. Mutating the Baxα mitochondria-targeting signal [L26P/L27P] appears to introduce a kink into helix α1. Protein–protein docking suggests that helices α9 of both wild-type Bax∆2 and Bax∆2 caspase 8 binding-deficient mutant [L164P] can fit in the same caspase 8 binding site, but the mutant is unable to fit as well as wild-type Bax∆2. Together, these data point to a structural basis for explaining Bax∆2 function in caspase 8-dependent cell death.

Unconventional Source of Neurotoxic Protein Aggregation from Organelle Off-Target Bax∆2 in Alzheimer’s Disease

“…The Baxα C-terminal tail contains a transmembrane domain for anchoring on mitochondria. When Bax proteins fail to target mitochondria and form aggregates, these tails are accessible for binding to the caspase 8 N-terminal death effector domain in cancer cells [ 21 ]. We demonstrated that the α9 helical structure rather than the primary sequence was critical for Bax∆2 binding to caspase 8 [ 19 ].…”

“…Can Bax∆2 still cause cell death if it cannot target mitochondria? We have uncovered the mechanism of Bax∆2-mediated cell death using both wet and dry lab approaches [ 19 , 20 , 21 ]. Although Bax∆2 did not target mitochondria due to the lack of exon 2 encoding helix α1, it contains an intact core region capable of oligomerization [ 12 ].…”

“…Although Bax∆2 did not target mitochondria due to the lack of exon 2 encoding helix α1, it contains an intact core region capable of oligomerization [ 12 ]. As a result, Bax∆2 proteins formed large protein aggregates in the cytosol which served as a platform to recruit caspase 8 using the C-terminal helix α9 (the mitochondrial anchoring domain for Baxα) [ 19 , 21 ]. Binding of caspase 8 to Bax∆2 aggregates triggered caspase-mediated but mitochondria-independent cell death.…”

“…Notably, the Bax∆2 C-terminal helix α9 acted as a “bridge” between Bax∆2 aggregates and caspase 8 interaction. Without the helix α9, Bax∆2 protein still formed aggregates, but such aggregates could not interact with caspase 8 to trigger cell death [ 19 , 21 ]. This suggests that protein aggregation alone is essential but not sufficient for the execution of cell death.…”

Detection of melanin-mediated false-positive for BaxΔ2 immunohistochemical staining in human skin tissues

“…Compared to the parental Baxα, Bax∆2 has a stronger pro-death potency [8]. Unlike most members of the pro-apoptotic Bax family which trigger apoptosis through the mitochondrial pathway, Bax∆2 proteins are unable to target mitochondria, instead they accumulate in the cytosol as protein aggregates, activating caspase 8dependent apoptosis [9]- [11]. Expression of Bax∆2 protein in human tissues has been well characterized by immunohistochemistry with anti-Bax∆2 antibody [12], [13].…”