Cytosolic Bax: does it require binding proteins to keep its pro-apoptotic activity in check?

Area of Science:

• Cell Biology
• Molecular Biology
• Apoptosis Regulation

Background:

• Bax protein, crucial for apoptosis, is maintained inactive in the cytosol.
• Energetic models predict inactive Bax stability, but numerous binding proteins have been proposed.
• Previous studies lacked validation of Bax binding proteins in physiological contexts.

Purpose of the Study:

• To investigate the physiological state of endogenous inactive Bax in the cytosol.
• To identify and validate proteins that bind to and stabilize inactive Bax.
• To clarify the mechanisms regulating Bax pro-apoptotic activity in healthy cells.

Main Methods:

• Gel filtration analysis of cellular cytosol.
• Blue native gel electrophoresis.
• High-sensitivity mass spectrometry following anti-Bax immunoprecipitation.
• SDS-PAGE analysis.
• Overexpression and underexpression studies of candidate binding proteins.

Main Results:

• Endogenous inactive Bax exhibited a higher molecular mass (20-30 kDa) than recombinant Bax in gel filtration.
• Bax dimerization was excluded through co-immunoprecipitation and blue native gel electrophoresis.
• Mass spectrometry identified p23 hsp90 co-chaperone as a specific Bax-binding protein.
• Neither p23 hsp90 manipulation nor Bax post-translational modifications affected apoptosis.
• Cytosolic Bax showed a slight molecular mass shift on SDS-PAGE compared to recombinant Bax.

Conclusions:

• Endogenous inactive Bax is likely monomeric in healthy cells.
• Bax does not require a binding partner for maintaining its inactive state.
• Post-translational modifications or structural differences may exist in cytosolic Bax.
• The proposed binding partners do not play a crucial role in inhibiting Bax activity.