2.A.38.4.2
High affinity (Km <50 μM) KtrA binds ATP to a β-α-β-Rossman fold. Other nucleotides bound, but only ATP bound with high affinity and changed the conformation of KtrA so that KtrA and KtrB associate (Kroning et al., 2007) in vivo. Both ATP and ΔΨ were required for transport activity. K⁺ uptake may occur by a Na⁺:K⁺ symport mechanism.  However, one region of KtrB, the transmembrane helix (M(2C); 40aas long) seems to control K⁺ transport non-coordinately with Na⁺ symport (Haenelt et al., 2010). M2C2 may form a flexible "gate" controlling K⁺ translocation at the cytoplasmic side of KtrB (Hänelt et al. 2010).  M(2C2) seems to be required for the interaction between KtrA and KtrB.