The ATP-dependent protease Ti consists of two different components: ClpA containing ATP-cleaving sites and ClpP having serine active sites for proteolysis. The clpA gene has dual translational start sites and therefore encodes two polypeptides with sizes of 84 and 65 kDa (referred to as ClpA84 and ClpA65, respectively). Here we show that ClpA84, but not ClpA65, is degraded in vitro by ClpP in the presence of ATP. The ClpP-mediated hydrolysis of ClpA84 could be prevented by casein, which is an excellent substrate of protease Ti (i.e. ClpA84/ClpP complex). Thus, it appears that free form of ClpA84 competes with casein for the degradation by ClpA/ClpP complex. Furthermore, ClpA65 inhibited the auto-degradation of ClpA84 by the complex. These results suggest that ClpA65 may play an important role in the control of the ClpA84 level and in turn in the regulation of ATP-dependent protein breakdown in E. coli.