CLPX, AN ALTERNATIVE SUBUNIT FOR THE ATP-DEPENDENT CLP PROTEASE OF ESCHERICHIA-COLI - SEQUENCE AND IN-VIVO ACTIVITIES

The ATP-dependent Clp protease of Escherichia coli consists of two sub units, the ClpP subunit, which has the proteolytic active site, and Cl pA, which possesses ATPase activity and activates the proteolytic acti vity of ClpP in vitro. Recently, Zylicz and co-workers (Wojtkowiak, D. , Georgopoulos, C., and Zylicz, M. (1993) J. Biol. Chem. 268, 22609-22 617) identified another E. coli protein that activated ATP-dependent d egradation of lambdaO protein in the presence of ClpP. The amino-termi nal sequence of this protein corresponds to the translated amino-termi nal sequence of a gene that we have named clpX. clpX encodes a protein with M(r) 46,300, similar to that observed for the protein purified b y Wojtkowiak et al. clpX is in an operon with clpP; both genes are cot ranscribed in a single heat-inducible 2200-base mRNA, with clpP the pr omoter proximal gene. The sequence of ClpX includes a single consensus ATP-binding site motif and has limited homology to regions of ClpA an d other members of the ClpA/B/C family. A third group of proteins, Clp Y, closely related to ClpX, has been identified by sequence homology. Mutations in either clpX or clpP abolish degradation of the highly uns table lambdaO protein in vivo. clpX mutants are not defective in degra dation of previously identified ClpA/ClpP substrates such as a ClpA-be ta-galactosidase fusion protein. It appears that selectivity of degrad ation by ClpP in vivo is determined by interaction of ClpP with differ ent regulatory ATPase subunits.