Energy Sensing versus 2-Oxoglutarate Dependent ATPase Switch in the Control of Synechococcus PII Interaction with Its Targets NAGK and PipX

P-II proteins constitute a superfamily of highly conserved signaling devices, common in all domains of life. Through binding of the metabolites ATP, ADP and 2-oxoglutarate (2-OG), they undergo conformational changes which allow them to regulate a variety of target proteins including enzymes, transport proteins and transcription factors. But, in reverse, these target proteins also modulate the metabolite sensing properties of P-II, as has been recently shown. We used this effect to refine our P-II based Forster resonance energy transfer (FRET) sensor and amplify its sensitivity towards ADP. With this enhanced sensor setup we addressed the question whether the P-II protein from the model organism Synechococcus elongatus autonomously switches into the ADP conformation through ATPase activity as proposed in a recently published model. The present study disproves ATPase activity as a relevant mechanism for the transition of P-II into the ADP state. In the absence of 2-OG, only the ATP/ADP ratio and concentration of ADP directs the competitive interaction of P-II with two targets, one of which preferentially binds P-II in the ATP-state, the other in the ADP-state.