Characterization of a specificity factor for an AAA+ ATPase: assembly of SspB dimers with ssrA-tagged proteins and the ClpX hexamer.

SspB, a specificity factor for the ATP-dependent ClpXP protease, stimulates proteolysis of protein substrates bearing the ssrA degradation tag. The SspB protein is shown here to form a stable homodimer with two independent binding sites for ssrA-tagged proteins or peptides. SspB by itself binds to ClpX and stimulates the ATPase activity of this enzyme. In the presence of ATPgammaS, a ternary complex of SspB, GFP-ssrA, and the ClpX ATPase was sufficiently stable to isolate by gel-filtration or ion-exchange chromatography. This complex consists of one SspB dimer, two molecules of GFP-ssrA, and one ClpX hexamer. SspB dimers do not commit bound substrates to ClpXP degradation but increase the affinity and cooperativity of binding of ssrA-tagged substrates to ClpX, facilitating enhanced degradation at low substrate concentrations.