Activity of E. coli ClpA bound by nucleoside diphosphates and triphosphates.

The Escherichia coli ClpA protein is a molecular chaperone that binds and translocates protein substrates into the proteolytic cavity of the tetradecameric serine protease ClpP. In the absence of ClpP, ClpA can remodel protein complexes. In order for ClpA to bind protein substrates targeted for removal or remodeling, ClpA requires nucleoside triphosphate binding to first assemble into a hexamer. Here we report the assembly properties of ClpA in the presence of the nucleoside diphosphates and triphosphates ADP, adenosine 5'-[γ-thio]triphosphate, adenosine 5'-(β,γ-imido)triphosphate, β,γ-methyleneadenosine 5'-triphosphate, and adenosine diphosphate beryllium fluoride. In addition to examining the assembly of ClpA in the presence of various nucleotides and nucleotide analogues, we have also correlated the assembly state of ClpA in the presence of these nucleotides with both polypeptide binding activity and enzymatic activity, specifically ClpA-catalyzed polypeptide translocation. Here we show that all of the selected nucleotides, including ADP, promote the assembly of ClpA. However, only adenosine 5'-[γ-thio]triphosphate and adenosine 5'-(β,γ-imido)triphosphate promote the formation of an oligomer of ClpA that is active in polypeptide binding and translocation. These results suggest that the presence of γ phosphate may serve to switch ClpA into a conformational state with high peptide binding activity, whereas affinity is severely attenuated when ADP is bound.