title: Energy-dependent degradation: Linkage between ClpX-catalyzed nucleotide hydrolysis and protein-substrate processing.
abstract: ClpX requires ATP to unfold protein substrates and translocate them into the proteolytic chamber of ClpP for degradation. The steady-state parameters for hydrolysis of ATP and ATPgammaS by ClpX were measured with different protein partners and the kinetics of degradation of ssrA-tagged substrates were determined with both nucleotides. ClpX hydrolyzed ATPgammaS to ADP and thiophosphate at a rate (6/min) significantly slower than ATP hydrolysis (140/min), but the hydrolysis of both nucleotides was increased by ssrA-tagged substrates and decreased by ClpP. K(M) and k(cat) for hydrolysis of ATP and ATPgammaS were linearly correlated over a 200-fold range, suggesting that protein partners largely affect k(cat) rather than nucleotide binding, indicating that most bound ATP leaves the enzyme by hydrolysis rather than dissociation, and placing an upper limit of approximately 15 micro M on K(D) for both nucleotides. Competition studies with ClpX and fluorescently labeled ADP gave inhibition constants for ATPgammaS ( approximately 2 micro M) and ADP ( approximately 3 micro M) under the reaction conditions used for steady-state kinetics. In the absence of Mg(2+), where hydrolysis does not occur, the inhibition constant for ATP ( approximately 55 micro M) was weaker but very similar to the value for ATPgammaS ( approximately 45 micro M). Compared with ATP, ATPgammaS supported slow but roughly comparable rates of ClpXP degradation for two Arc-ssrA substrates and denatured GFP-ssrA, but not of native GFP-ssrA. These results show that the processing of protein substrates by ClpX is closely coupled to the maximum rate of nucleotide hydrolysis.
authors.affiliation: Department of Biology, Howard Hughes Medical Institute, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Randall E
authors.initials: RE
authors.lastname: Burton
authors.forename: Tania A
authors.initials: TA
authors.lastname: Baker
authors.forename: Robert T
authors.initials: RT
authors.lastname: Sauer
mesh_terms: D000074183:ATPases Associated with Diverse Cellular Activities; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D001667:Binding, Competitive; D002384:Catalysis; D049071:Endopeptidase Clp; D029968:Escherichia coli Proteins / Q000378:metabolism; D006868:Hydrolysis; D007700:Kinetics; D018832:Molecular Chaperones; D009711:Nucleotides / Q000378:metabolism*; D010447:Peptide Hydrolases / Q000378:metabolism; D011489:Protein Denaturation; D021381:Protein Transport; D012697:Serine Endopeptidases / Q000378:metabolism
chemical_list: D029968:Escherichia coli Proteins; D018832:Molecular Chaperones; D009711:Nucleotides; D000255:Adenosine Triphosphate; D010447:Peptide Hydrolases; D012697:Serine Endopeptidases; C483573:ClpXP protease, E coli; D049071:Endopeptidase Clp; D000251:Adenosine Triphosphatases; C088072:ClpX protein, E coli; D000074183:ATPases Associated with Diverse Cellular Activities