Part of the Na(+)-dependent high affinity K(+) uptake system KtrAB. KtrA is the regulatory subunit and plays an important role in the substrate specificity and transport mechanism of the system. Binds ATP but lacks ATPase activity. Requires both ATP and a high membrane potential for activity. Binding of ATP causes a conformational change in KtrA, which promotes formation of the KtrAB complex. Can also bind, with lower affinity, other nucleotides such as NADH or NAD(+), but only ATP can induce a conformational change. The uptake system is composed of KtrA and KtrB. Belongs to the KtrA potassium transport family.