Involved in regulation of glutamate metabolism. Acts as a phosphorylation-dependent molecular switch that modulates the activities of Kgd, Gdh and GltB. Phosphorylation triggers an intra-molecular protein closure, which blocks the FHA binding site and interaction with target enzymes, and switches off the regulatory function of GarA. Monomer. Binds via its FHA domain to Kgd, Gdh, GltB, PknB, and the N-terminal region of PknG. Phosphorylated on Thr-22 by PknB. Phosphorylated on Thr-21 by PknG. Phosphorylation at either Thr-21 or Thr-22 prevents binding to target enzymes. Phosphorylation at these two threonines is mutually exclusive in vitro. Could also be phosphorylated by PknD, PknE and PknF.