In this application, we focus on determining the kinetic details of the ATP-dependent protease Lon by addressing two specific questions: 1) how does the timing of ATP binding and hydrolysis affect the catalytic efficiency of unfolded protein degradation, and 2) What are the substrate determinants of the cleavage sites? Since the rate of cellular protein degradation is dependent on the catalytic efficiency of ATP-dependent proteases, it is important to investigate how these enzymes coordinate ATP binding and hydrolysis with peptide cleavage to obtain maximal protein degradation efficiency. Based upon steady-state velocity and product inhibition as well as preliminary pre-steady state kinetic analyses, we propose that ATP hydrolysis occurs prior to peptide cleavage, and the rate-limiting step for peptide degradation should exhibit dependence on ATP hydrolysis. Since pre-steady state kinetic techniques allow one to determine the microscopic rate constants associated with the ATPase and the peptidase reactions, we will employ this technique to establish the sequence of events occurring along the Lon reaction pathway. To gain insight into the relationship between ATP hydrolysis and processive proteolysis, we will evaluate how Lon cleaves polypeptide substrates containing multiple cleavage sites. In addition, we will determine the energetic requirement of Lon cleaving a defined peptide substrate that adopts a helical conformation upon binding to RNA by assessing whether ATP hydrolysis is required for unfolding as well as for hydrolysis. We will also pursue steady-state kinetic characterization of the two mammalian Lon (mt Lon) proteases using the synthetic peptide FRETN 89-98 as substrate to evaluate the mechanistic similarities between E. coli and mt Lon. Furthermore, we will characterize the in vitro degradation of b F 1-ATPase by human and mouse Lon to evaluate the functional relationship between mt Lon and F1-ATPase degradation to obtain insight into the role played by Lon in rendering mitochondria function.