Extended Data Figure 7. Compact assembly of the PA pore.
a, Surface views of two neighboring protomers (the same domains in the two protomors are in different shades of the same colors) of the PA pore and the PA prepore (PDB ID: 1TZO) visualized from inside PA heptamer. Domains 1', 2c, 2s, and 3 are colored differently and domain 4 is not shown. The inter-protomer interface in the PA prepore is largely formed by domains 1' and 3 and domain 2 only contributes to this interface by its membrane insertion loop and C-terminal region (2β13, 2β14, and 2α3). In the PA pore, the convergence of domain 2c creates an inter-protomer interface without any gap, with an increase of interface area on domain 2c from 1,247 Å2 of the PA prepore to 2,106 Å2 of the PA pore as calculated using the PISA49 (http://www.ebi.ac.uk/pdbe/pisa/). Additionally, formation of the β barrel also leads to extensive contacts, creating a new interface area of 1,195 Å2 between two protomers. b, Schematic of the conversion of 2β2, 2β3, 2α1, and the membrane insertion loop of the PA prepore to 2β2s and 2β3s of the PA pore. Hydrogen bonds between 2β2 and 2β3, which are depicted with dashed lines, are maintained during the conversion. By contrast, 2α1 and the connecting loops have to be fully unfolded and converted into β strands that collectively assemble the β barrel of the PA pore. Although the detailed events of β barrel formation are not yet clear, it is likely that assembly starts from the top in a zipper-like manner. A favorable scenario is that the convergence of domains 2c would place the top ends of the β strands close to each other to form a short β barrel, which could extend by pulling more residues together via formation of ordered hydrogen bonds until it reaches the bottom end. It is less favorable that the assembly starts from other regions because disordered hydrogen bonds and hydrophobic interactions could generate enormous non-productive, possibly irreversible pairings between strands.