20S human proteasomes bind with a specific orientation to lipid monolayers in vitro

Area of Science:

• Biochemistry
• Cell Biology
• Structural Biology

Background:

• 20S proteasomes are key proteinases involved in degrading misfolded and regulatory proteins.
• They form the core of the 26S proteasome complex, essential for the ubiquitin-dependent proteolytic pathway and antigen processing.
• While typically degrading unfolded proteins, 20S proteasomes are known to associate with cellular membranes.

Purpose of the Study:

• To investigate the in vitro association of 20S proteasomes with lipid membranes.
• To determine the structure and orientation of 20S proteasomes bound to lipid monolayers.
• To understand the preferred binding sites and orientation of 20S proteasomes on different phospholipid compositions.

Main Methods:

• Utilized a lipid monolayer system as a model for biological membranes.
• Employed electron microscopy to analyze the structure and orientation of bound proteasomes.
• Tested association with various phospholipids including PI, PA, PG, PS, PC, and mitochondrial lipids.

Main Results:

• 20S proteasomes exhibited an "end-on" configuration specifically on phosphatidylinositol (PI) lipid monolayers, forming large arrays.
• Proteasomes were oriented with their channels facing away from the lipid headgroups on PI monolayers.
• Similar orientation was observed on ER and Golgi lipid films, while random orientation occurred with other tested lipids.

Conclusions:

• 20S proteasomes bind to phospholipids in vitro with a preferred, specific orientation.
• This orientation positions the proteasome channel perpendicular to the membrane surface.
• The findings provide insights into the structural basis of proteasome-membrane interactions.