Metabolically regulated proteasome supramolecular organization in situ

Area of Science:

• Cell Biology
• Structural Biology
• Biochemistry

Background:

• Membraneless organelles are crucial for cellular regulation.
• Understanding their formation and structural dynamics is limited by low-resolution intracellular data.

Purpose of the Study:

• To investigate the in situ formation and structure of yeast proteasome storage granules (PSGs).
• To elucidate the structural rearrangements of proteasomes during organelle assembly.

Main Methods:

• In situ cryo-electron tomography (cryo-ET) was employed to visualize PSG formation in yeast.
• High-resolution (9-Å) cryo-ET structures were obtained under various energy conditions.

Main Results:

• Doubly capped 26S proteasomes form inactive trimeric units during the proliferation-to-quiescence transition.
• Cytoplasmic PSGs assemble as paracrystalline arrays of bundled fibers from stacked proteasome trimers.
• This arrangement allows for the storage and subsequent release of inactive proteasomes based on energy availability.

Conclusions:

• The study reveals key structural steps in the in situ assembly of a membraneless organelle.
• Quinary structure formation of proteasomes within PSGs controls this major eukaryotic regulatory machine.