ROLE OF THE HEAT-SHOCK PROTEIN DNAJ IN THE LON-DEPENDENT DEGRADATION OF NATURALLY UNSTABLE PROTEINS

We have investigated the role of DnaJ in protein degradation by examin ing the degradation of intrinsically unstable proteins by Lon protease in vivo. In Escherichia coil, Lon protease is responsible for the rat e-limiting step in degradation of highly unstable proteins such as Sul A, RcsA, and lambda N protein, as well as for about 50% of the rapid d egradation of abnormal proteins such as canavanine-containing proteins . We found that Lon-dependent degradation of both SulA and AN protein was unaffected in cells lacking functional DnaJ, whereas Lon-dependent turnover of canavanine containing proteins was slower in dnaJ mutant cells. DnaJ also affected the slow SulA degradation seen in the absenc e of Lon. The rate of degradation of RcsA was reduced in dnaJ mutants, but both Lon-dependent and Lon-independent degradation was affected; abnormal, canavanine-containing proteins were similarly affected. Both the RcsA that accumulated in dnaJ mutant cells and the SulA that accu mulated in ion dnaJ mutant cells was aggregated. The abnormal proteins that partitioned to the insoluble pellet became solubilized over time in dnaJ(+) cells but not in dnaJ(-) cells. Therefore, the co-chaperon e DnaJ is not essential for Lon-dependent degradation and may act in p rotein turnover only as an accessory factor helping to maintain substr ates in a soluble form. Such an accessory factor is apparently necessa ry for abnormal proteins and for RcsA. The relative rates of degradati on and aggregation of specific protein targets may determine the impor tance of the chaperone systems in turnover of a given protein.