Mitochondrial Lon of Saccharomyces cerevisiae is a ring-shaped protease with seven flexible subunits

Lon (or La) is a soluble, homooligomeric ATP-dependent protease. Mass deter mination and cryoelectron microscopy of pure mitochondrial Lon from Sacchar omyces cerevisiae identify Lon as a flexible ring-shaped heptamer. In the p resence of ATP or 5'-adenylylimidodiphosphate, most of the rings are symmet ric and resemble other ATP-driven machines that mediate folding and degrada tion of proteins. In the absence of nucleotides, most of the rings are dist orted, with two adjacent subunits forming leg-like protrusions. These resul ts suggest that asymmetric conformational changes serve to power processive unfolding and translocation of substrates to the active site of the Lon pr otease.