Fig. 4.

Models for allosteric control of Lon. (A) Model showing equilibrium states of a system with three Lon conformations: Lon^OFF, Lon^ON, and Lon^DEG (depicted by different shapes). Degrons are shown as squiggly lines. One degron-binding site in the axial pore of Lon and two flanking allosteric binding sites are depicted. At substrate saturation, the relative affinity of the degron for Lon^ON versus Lon^DEG determines the equilibrium distribution of the two fully liganded species shown in the rectangle. (B) Model in which an unfolded protein binds Lon and then partitions between a degradation pathway mediated by Lon^DEG or a refolding pathway mediated by Lon^ON and possibly other cellular chaperones. The substrate degron determines the equilibrium distribution of the two forms of Lon and thus determines which pathway is favored.