The isolated, twenty-three-residue-long, N-terminal region of the glutamine synthetase inactivating factor binds to its target

•The region Ala7-Ala29 (IF7pep) of the inactivating factor (IF7) of GS binds to GS.•The binding of IF7pep to GS occurs with the same affinity as that of intact IF7.•Isolated IF7pep has an intrinsic tendency to populate α-helix-like conformations.•Isolated IF7pep does not inactivate GS.•The binding reaction is electrostatically-driven involving several species.

Glutamine synthetase (GS) catalyzes the ATP-dependent formation of glutamine from glutamate and ammonia. The activity of Synechocystis sp. PCC 6803 GS type I is regulated by protein-protein interactions with a 65-residue-long protein (IF7). IF7 binds initially to GS through residues at its N terminus. In this work, we studied the conformational preferences of the N-terminal region of IF7 (IF7pep, residues Ala7-Ala29), its binding to GS and its functional properties. Isolated IF7pep populated a nascent helix in aqueous solution. IF7pep was bound to GS with an affinity constant of 0.4 μM, and a 1:1 stoichiometry. IF7pep did not inactivate GS, suggesting that there were other IF7 regions important to carry out the inactivating function. Binding of IF7pep to GS was electrostatically-driven and it did not follow a kinetic two-state model.

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