Selectivity of intracellular proteolysis: protein substrates activate the ATP-dependent protease (La).

A critical enzyme in protein breakdown in Escherichia coli is protease La (the lon gene product), which hydrolyzes proteins and adenosine triphosphate (ATP) in a coupled process. The mechanism of this process was studied with fluorogenic tripeptides. Although proteins and peptides are degraded at the same active site, protein substrates enhance the ability of the enzyme to degrade these peptides two- to tenfold. Proteins that are not substrates had little or no effect. Thus, protein substrates must bind to protease La at two sites, the active site and an allosteric site whose occupancy enhances proteolytic activity. This effect did not require that the proteins themselves be degraded. Proteins could induce peptide breakdown even in the absence of ATP, and proteins and ATP had additive effects in stimulating peptidase activity. A multistep cyclical mechanism is proposed in which the binding of the substrate and ATP activates the protease. The enzyme can then cleave a peptide bond, but is inactivated through ATP hydrolysis. Such a mechanism may help account for the selectivity of protein breakdown and prevent inappropriate or excessive proteolysis in vivo.