Fig. 3: POMP and β2 displaces PAC3/PAC4 to enable β-ring assembly.

a Cryo-EM map of PAC1-4/α-ring overlayed with the pre-13S atomic model. POMP fits into the space between PAC3/PAC4 and α1-2. β2 and β3 bind to the α-ring on the opposite side of PAC3/PAC4. Scale bar: 15 Å. b Isolated views of the overlayed structures show good fit between POMP and PAC3/PAC4 with little overlap between the chaperones. Scale bar: 15 Å. c The PAC4 structure clashes with the C-terminal region of POMP and the N-terminal tail of the β2 pro-peptide (arrowheads), which are stabilized by interactions with β3. Scale bar: 5 Å. d Schematic depicting the initiation of β-ring assembly and dissociation of PAC3/PAC4. e Comparison of our PAC1-4/α-ring complex with the structure of PAC1-4/α-ring/POMP/β2 (PDB 8QYJ) shows that binding of POMP disrupts the interaction between PAC3 and α7. Scale bar: 3 Å.