Characterization of protein complexes involved in signal transduction of chemotaxis in Escherichia coli

Chemotaxis proteins are responsible for transmitting signals received from ligand-bound transmembrane receptors to the flagellar motor to control the direction of flagellar rotation. The direction of rotation of the flagella determines whether the bacterium will swim in a forward direction or tumble to reorient itself in a more favorable direction. Control of the rotation is the basis for chemotactic response.;Protein-protein interactions involved in signal transduction during bacterial chemotaxis were investigated. Protein affinity chromatography and immunoprecipitation were used to demonstrate the characterize three protein complexes.;A complex of CheA76/CheA66/CheW with the CheY protein was characterized. These proteins interacted with CheY in a 1:1:1 ratio. CheW and at least one species of CheA protein were required for interaction with CheY. This complex was postulated to be a means of enhancing phosphate transfer from CheA76 to CheY to activate CheY, which is the tumble regulator in the cell. CheW enhances phosphate transfer from CheA76 to CheY by increasing the affinity of CheA76 for CheY.;A distinct interaction between CheZ and CheY proteins was also shown. These proteins interacted with each other in the absence of CheA and CheW proteins. CheZ had previously been shown to be a CheY phosphatase, resulting in deactivation of the tumble regulator. In this study, it was proposed that CheZ inactivates CheY by direct interaction with CheY, resulting in removal of phosphate. Finally, a complex between CheA66, CheW and CheZ proteins was identified and characterized. CheW protein interacted with CheZ only in the presence of CheA66. However, CheA66 bound to CheZ in the absence of CheW. Thus, CheW binds to CheZ through its interaction with CheA66, but is not required for CheA66 interaction with CheZ. The role of this complex as either being involved in transmitting a signal to attractant stimulus or in amplifying a negative response in chemotaxis and the roles of the other complexes identified were discussed in a working model.