BAX mitochondrial integration is regulated allosterically by its α1-α2 loop

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• BAX protein activation is a critical step in apoptosis, involving conformational changes from a cytosolic monomer to homo-oligomers.
• These oligomers permeabilize the mitochondrial outer membrane (MOM), initiating programmed cell death.
• The BAX α1-α2 loop's role in this activation process remains incompletely understood.

Purpose of the Study:

• To investigate the functional significance of the BAX α1-α2 loop in BAX-mediated apoptosis.
• To determine how specific mutations within the α1-α2 loop affect BAX conformational changes and MOM integration.
• To explore the implications of α1-α2 loop mutations in cancer.

Main Methods:

• Utilized three mutagenic approaches: loop region replacement, alanine scanning, and analysis of naturally occurring tumor mutations.
• Assessed the responsiveness of BAX variants to death signals like tBID.
• Evaluated the impact of mutations on MOM integration and BAX stability.

Main Results:

• Mutations in the N-terminal half of the α1-α2 loop reduced responsiveness to death signals and impaired MOM integration by allosterically affecting the α9 transmembrane anchor.
• N-terminal loop variants, found enriched in tumors, were shown to hinder MOM integration.
• Most C-terminal loop variants decreased BAX stability, with some variants exhibiting enhanced apoptotic function.

Conclusions:

• The N-terminal and C-terminal halves of the BAX α1-α2 loop possess distinct functions in apoptosis regulation.
• The N-terminal half acts as an allosteric regulator of BAX activation, controlling MOM integration post-death signal.
• Enrichment of N-terminal loop mutations in tumors suggests a role in tumor cell survival and highlights the loop as a therapeutic target.