# Structural transitions in the stepwise assembly of proteasome core particles

## TL;DR

This study reveals how proteasome core particles assemble through multiple pathways, with chaperones playing key roles in the process.

## Contribution

The paper shows that proteasome assembly in yeast involves alternative and potentially simultaneous pathways, not a single fixed trajectory.

## Key findings

- Cryo-EM structures reveal alternative assembly pathways for proteasome core particles in yeast.
- Ump1 and Pba1-Pba2 chaperones help structure β-subunits and prepare peptidase sites for activation.
- Pba1 interacts transiently with the α-ring, coordinating maturation and chaperone release.

## Abstract

20S catalytic core particles (CP) of eukaryotic 26S proteasomes are composed of two identical halves comprising 14 distinct subunits. 15S precursor complexes (PC) represent detectable half-CPs assembly intermediates lacking the β7-subunit but containing assembly chaperones Ump1 and Pba1-Pba2. Incorporation of β7 drives 15S-PC dimerisation and further CP maturation. Our cryo-EM structures of the yeast 15S-PC and all 13S-PC-derived intermediates suggest that assembly in yeast is not restricted to a single trajectory, but instead involves alternative, and potentially simultaneous pathways. Comparison of the intermediates reveals how Ump1 and β-subunits become structured with each additionally incorporated β-subunit, and how this prepares peptidase sites for auto-activation. We identify two transient interactions of Pba1 with the α-ring, which are important for an ordered progression of maturation. Pba1 loop 81-117 intercalates between subunits α3 and α4 in 13S-15S-PCs and is displaced upon 15S-PC dimerisation. The second interaction involves the α1 N-terminus, deletion of which leads to a defect in Pba1-Pba2 release. These findings indicate how changes in α-ring subunit conformations coordinate CP maturation with Pba1‑Pba2 release.

Eukaryotic 20S proteasomes assemble from 14 distinct subunits. Here, the authors use cryo-EM to analyze multiple intermediates showing that biogenesis involves alternative routes, rather than one fixed pathway. They also reveal roles of Ump1 and Pba1-Pba2 chaperones in orchestrating maturation.