Bifurcated assembly pathway and dual function of a Lon-like protease revealed by cryo-EM Analysis

The Lon proteases are evolutionarily conserved oligomeric hydrolases characterized by a built-in ATPase module. Here, we report the structures of LonC from Meiothermus taiwanensis (MtaLonC), a unique member in the Lon protein family with a dual chaperone-protease function but lacking ATPase activity, determined by cryo-electron microscopy (cryo-EM). In its apo state, LonC forms phosphate-bound open-ring pentamers, open-ring hexamers, and close-ring heptamers. However, upon interaction with ATPγS, inhibitor, or substrate, MtaLonC assembles into close-ring hexamers, wherein the protease domain's substrate-binding loop adopts an extended active conformation. We show that proteolytic and chaperone activities may be carried out by the close-ring hexameric and heptameric forms, respectively. We further show that MtaLonC forms exclusively close-ring heptamers at elevated temperatures. This study sheds light on the bifurcated assembly pathway of MtaLonC, leading to two distinct oligomeric close-ring complexes that carry out a dual function.