Abrahams, J. P.; Ahrensback Roesgaard, M.; Armbruster, P.; Sharpe, T.; Schenck, N.

The mitochondrial Lon protease is essential for proteostasis through ATP-dependent proteolysis and suppression of protein aggregation through an unknown mechanism. Here we show in three independent aggregation systems that human Lon protease (LonP1) directly interacts with fibrillar aggregates to prevent further aggregation: LonP1 binds amyloid fibrils and inhibits their growth, independently of its protease and ATPase activities. This aggregation inhibition depends on hexamer stability, and even the N-domain hexamer of LonP1 lacking all catalytic domains inhibited aggregation, which localizes its fibril-binding interface. We propose that chaperone deficiencies in LonP1 mutants that are associated with genetic disease, are caused by reduced hexamer stability or increased turnover. Our results clarify the observed dual protease and chaperone function of LonP1 by localizing them to different domains and separating the catalytic activities, thereby facilitating targeting the specific functionalities. Further, we identify the structure of the chaperone substrate to be fibrillar aggregates, suggesting that LonP1 may protect against amyloid fibrils in healthy individuals.