A ubiquitin-independent proteasome pathway controls activation of the CARD8 inflammasome

Area of Science:

• Immunology
• Molecular Biology
• Cellular Biology

Background:

• CARD8 (Caspase recruitment domain-containing protein 8) is a pattern-recognition receptor crucial for inflammasome activation.
• CARD8 undergoes autoproteolysis, yielding N-terminal (NT) and C-terminal (CT) fragments, with the NT fragment regulating inflammasome assembly.

Purpose of the Study:

• To elucidate the role of the 20S proteasome in regulating CARD8 inflammasome activation.
• To investigate the mechanism by which proteasome-mediated degradation controls CARD8 function.

Main Methods:

• Utilized biochemical assays to study protein degradation pathways.
• Employed proteasome activity assays in the presence and absence of specific inhibitors (Val-boroPro).
• Analyzed protein fragments and domain structures using techniques like mass spectrometry and structural biology.

Main Results:

• The core 20S proteasome degrades the disordered region of the CARD8 NT fragment in unstressed cells, maintaining an autoinhibited state.
• In cells treated with dipeptidyl peptidase inhibitors (e.g., Val-boroPro), the 20S proteasome degrades the entire NT fragment, potentially due to ZU5 domain unfolding.
• This complete degradation releases the CT fragment, leading to inflammasome activation.

Conclusions:

• The 20S proteasome's selective degradation of the CARD8 NT domain is a key regulator of inflammasome activation.
• The susceptibility of the CARD8 NT domain to proteasomal degradation, influenced by protein conformation, determines inflammatory signaling outcomes.