Structural basis of BAK sequestration by MCL-1 in apoptosis.

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Apoptosis is crucial for tissue homeostasis, and its dysregulation contributes to diseases like cancer.
• The BCL-2 (B-cell lymphoma 2) protein family regulates apoptosis, with effector proteins (BAK, BAX) mediating mitochondrial outer membrane permeabilization.
• Prosurvival BCL-2 proteins, such as MCL-1 (Myeloid Cell Leukemia 1), inhibit effector proteins, while BH3-only proteins activate them.

Purpose of the Study:

• To structurally and biochemically characterize the human prosurvival MCL-1:BAK complex.
• To evaluate the efficacy of existing MCL-1 inhibitors against this complex.
• To inform the design of novel, more effective MCL-1 inhibitors for cancer therapy.

Main Methods:

• Integrated structural biology (crystallography), biochemical assays, and pharmacological studies.
• Characterization of the MCL-1:BAK complex using BCL-2 globular core domains.
• Assessment of apoptosis induction by MCL-1 inhibitors in the presence of the MCL-1:BAK complex.

Main Results:

• Revealed a canonical interaction where BAK BH3 binds to the MCL-1 hydrophobic groove.
• Identified disordered and dynamic regions of BAK outside the core interaction interface.
• Demonstrated that current MCL-1 inhibitors are inefficient at neutralizing the MCL-1:BAK complex, requiring high concentrations to induce apoptosis.

Conclusions:

• The MCL-1:BAK complex structure provides insights into apoptosis regulation.
• Existing MCL-1 inhibitors show limited efficacy against the target complex.
• There is a critical need for developing superior clinical candidate MCL-1 inhibitors to effectively target cancer.