Fig. 6. Proposed model for the substrate-dependent activation of Lon.

Two diagrams depict the unwrapped and axial views of the AAA+ domains of the Lon complex in the open-spiral pentameric form (5S; center), the spiral hexameric form (6S; left), and the close-ring hexameric form (6C; right). The NGDs are shown of the Ob-protomers, colored in blue, which are linked by the tensegrity helix triangle (THT) scaffold (gray lines). The protomer Ac6* is highlighted in red. The diagrams illustrate how the binding of the substrate (S; orange) to the three NGDs and incorporation of Ac6* with Ob1 and Ob5 allosterically induce the formation of the substrate-pore-loop contacts in the universal right-handed staircase arrangement for substrate translocation. The pore-loop I and II residues accessible for substrate binding are denoted by hollow star and dot symbols, respectively. The pore-loop residues making substrate contacts are shown in solid symbols.