A dual allosteric pathway drives human mitochondrial Lon

The hexameric, barrel-forming, AAA+ protease Lon is critical for maintaining mitochondrial matrix protein homeostasis. Efficient substrate processing by Lon requires the coordinated action of six protomers. Despite Lon’s importance for human health, the molecular bases for Lon’s substrate recognition and processing remain unclear. Here, we use a combination of biochemistry and electron cryomicroscopy (cryo-EM) to unveil the structural and functional basis for full-length human mitochondrial Lon’s degradation of mitochondrial transcription factor A (TFAM). We show how opposing protomers in the Lon hexamer barrel interact through their N-terminal domains to give what resembles three feet above the barrel and help to form a triangular pore located just above the entry pore to the barrel. The interactions between opposing protomers constitute a primary allosteric regulation of Lon activity. A secondary allosteric regulation consists of an inter-subunit signaling element in the ATPase domains. By considering the ATP or ADP load in each protomer, we show how this dual allosteric mechanism in Lon achieves coordinated ATP hydrolysis and substrate processing. This mechanism enforces sequential anti-clockwise ATP hydrolysis resulting in a coordinated hand-over-hand translocation of the substrate towards the protease active sites.