Proteasomal Biogenesis

The 26S proteasome is a 2.5MDa protein complex tasked with timely degradation of polyubiquitinated proteins in eukaryotic cells. It is composed of a barrel-shaped 20S core particle (CP), which is associated with one or two 19S regulatory particles. The 20S CP consists of 7 distinctive a and 7 distinctive b subunits, assembled into stacked a7/b7/b7/a7 rings. 20S proteasome biogenesis is an ordered process aided by several chaperones, namely Pba1-4 and Ump1. The heterodimeric chaperones Pba1-2 and Pba3-4 promote a ring assembly and subsequent addition of the b2, b3, and b4 subunits displace Pba3-4. The next detectable intermediate is termed the 15S precursor complex and it additionally contains b5, b6, b1, and Ump1. Upon addition of b7, two 15S complexes rapidly dimerise and autocatalytic pro-peptide cleavage of the proteolytic subunits b1, b2 and b5 takes place. Our recent study elucidates the late steps of yeast 20S biogenesis using a combination of biochemical analysis, single particle EM as well as cross-linking and mass spectrometric analysis. We show that Ump1 is a largely unstructured protein that loops along the inside surface of the proteasome a and b rings and that Pba1-2 is embedded in the centre of the a ring in early proteasomal precursors. The Pba1-2 heterodimer is expelled by structural re-arrangements during proteasome maturation and is recycled rather than degraded in this process.