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<p>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.</p>

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Structural transitions in the stepwise assembly of proteasome core particles

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年3月24日

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  • Eric Mark,
  • Paula C. Ramos,
  • Maria M. Nunes,
  • Ana C. Matias,
  • R. Jürgen Dohmen,
  • Petra Wendler

摘要

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.