Structural transitions in the stepwise assembly of proteasome core particles

Area of Science:

• Cellular Biology
• Molecular Biology
• Structural Biology

Background:

• The 26S proteasome is a crucial cellular machine for protein degradation.
• Its 20S catalytic core particle (CP) assembles from multiple subunits.
• Assembly intermediates, like 15S precursor complexes (PCs), offer insights into CP formation.

Purpose of the Study:

• To elucidate the assembly pathways and structural dynamics of the 20S CP in yeast.
• To understand the role of assembly chaperones Ump1 and Pba1-Pba2 in CP maturation.
• To identify key interactions governing CP assembly and subunit incorporation.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine structures of yeast 15S-PCs and related intermediates.
• Comparative structural analysis of various assembly intermediates.
• Investigating the impact of specific mutations on chaperone release.

Main Results:

• Yeast 20S CP assembly proceeds through multiple, potentially simultaneous pathways, not a single trajectory.
• Structural analysis revealed how chaperones Ump1 and Pba1-Pba2 interact with forming CP subunits.
• Two specific transient interactions of Pba1 with the alpha-ring were identified, crucial for maturation and chaperone release.
• Deletion of the Pba1 loop or alpha1 N-terminus disrupted ordered maturation and chaperone release.

Conclusions:

• The study reveals a flexible assembly process for the 20S proteasome CP with alternative pathways.
• Chaperone interactions, particularly Pba1's transient binding to the alpha-ring, are critical for coordinated CP maturation.
• Conformational changes in the alpha-ring regulate the release of assembly chaperones, ensuring proper CP formation.