The contribution of individual domains of chloroplast protein AtDeg2 to its chaperone and proteolytic activities

The thylakoid protease AtDeg2 is a non-ATP hydrolyzing chloroplast protease/chaperone peripherally connected with stromal side of thylakoid membrane. Its linear structure consists of protease domain and two PDZ domains. To unveil the significance of individual domains, chaperone and proteolytic activities of AtDeg2, its mutated recombinant versions have been developed and their ability to suppress protein aggregation and resolubilization of protein aggregates as well as the ability to degrade substrate protein was examined in vitro. Our work reveals for the first time that AtDeg2 is able not only to suppress aggregation of denatured proteins, but to resolubilize existing protein aggregates as well. We show that PDZ2 domain contributes significantly to both chaperone and protease activities of AtDeg2, whereas PDZ1 is required for chaperone but superfluous for proteolytic activity. Protease domain – but not S-268 in its catalytic site – contributes to chaperone activities of AtDeg2. These results show an entirely new function of AtDeg2 chaperone/protease (i.e., disaggregation of protein aggregates) and allow to identify structural motifs required for “old” and new functions of AtDeg2.