Visual and functional demonstration of growing Bax-induced pores in mitochondrial outer membranes

Area of Science:

• Cell Biology
• Biochemistry
• Structural Biology

Background:

• Bax protein initiates mitochondrial outer membrane permeabilization (MOMP), a crucial event in apoptosis.
• MOMP involves the release of proteins from mitochondria to the cytoplasm, triggering cell death.
• Understanding the structural mechanism of Bax-induced MOMP is essential for deciphering apoptotic pathways.

Purpose of the Study:

• To visualize and characterize the structure of Bax-induced pores in mitochondrial outer membranes (MOMs).
• To elucidate the dynamic behavior and size evolution of these pores.
• To investigate the mechanism by which Bax facilitates the release of molecules through these pores.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to visualize Bax-induced pores in purified MOMs.
• Flow cytometry was used to monitor the release of fluorescent dextrans from isolated MOM vesicles.
• Analysis of pore size and release kinetics of molecules with varying molecular weights.

Main Results:

• Cryo-EM revealed solitary pores with negative curvature at their edges.
• Pore diameters expanded significantly over time, reaching up to 100-160 nm and even over 300 nm.
• Flow cytometry confirmed gradual release of dextrans, consistent with dynamically widening pores capable of releasing molecules from 10-500 kDa.

Conclusions:

• Bax induces the formation of large, dynamic pores in the mitochondrial outer membrane.
• These pores grow substantially over time, accommodating the release of diverse molecular sizes.
• The mechanism of Bax-mediated MOMP involves membrane-curvature stress and the formation of expanding pores.