Degradation of Lon in Caulobacter crescentus.

Protein degradation is an essential process in all organisms. This process is irreversible and energetically costly; therefore, protein destruction must be tightly controlled. While environmental stresses often lead to upregulation of proteases at the transcriptional level, little is known about posttranslational control of these critical machines. In this study, we show that in Caulobacter crescentus levels of the Lon protease are controlled through proteolysis. Lon turnover requires active Lon and ClpAP proteases. We show that specific determinants dictate Lon stability with a key carboxy-terminal histidine residue driving recognition. Expression of stabilized Lon variants results in toxic levels of protease that deplete normal Lon substrates, such as the replication initiator DnaA, to lethally low levels. Taken together, results of this work demonstrate a feedback mechanism in which ClpAP and Lon collaborate to tune Lon proteolytic capacity for the cell.IMPORTANCE Proteases are essential, but unrestrained activity can also kill cells by degrading essential proteins. The quality-control protease Lon must degrade many misfolded and native substrates. We show that Lon is itself controlled through proteolysis and that bypassing this control results in toxic consequences for the cell.