Fig. 4: Pre-20S formation triggers autocatalysis of the β pro-peptides.

a The cryo-EM map of mixed-13S shows poorly resolved density for the loops near the dimerization interface, indicating the region is likely flexible during β-ring assembly. Scale bar: 5 Å. b The sidechain of β2 K33 is not ordered in the mixed-13S structure, preventing cleavage of the β pro-peptide. Scale bar: 3 Å. c Dimerization of two half-proteasomes reorders the loops near the dimer interface into a stable structural arrangement. Scale bar: 5 Å. d Structural rearrangements induced by half-proteasome dimerization positions the sidechain of K33 into a catalytically active conformation. A break in the cryo-EM density (gray) can be observed between β2 T1 and T-2 at higher σ thresholds, indicating cleavage of the pro-peptide. Scale bar: 3 Å. e Quantification of the average cryo-EM map densities for the first six residues of the β pro-peptides after T1. Box plots indicate median (middle line), 25th, 75th percentile (box) and 5th and 95th percentile (whiskers) as well as outliers (single points). n = 6 residues. *p-value < 0.01, evaluated using two-tailed t-test with unequal variance. f Schematic depicting how structural rearrangement of K33 activates autocatalysis of the β pro-peptide. Scale bar for cryo-EM maps: 10 Å. Source data are provided as a Source data file.