Catalytic subunit of the KtrAB potassium uptake transporter. The 2 major potassium transporter complexes KtrAB and KtrCD confer resistance to both suddenly imposed and prolonged osmotic stress. Homodimer, tetramer (dimer of homodimer) and octamer (tetramer of homodimer). Part of the KtrAB complex formed by an octameric catalytic ring of KtrA and a membrane associated dimer of KtrB forming a potassium channel. Impaired potassium uptake. Belongs to the KtrA potassium transport family.