Part of the Na(+)-dependent high affinity K(+) uptake system KtrAB. KtrB is the K(+)-translocating subunit. K(+) transport is stimulated by Na(+). The uptake system is composed of KtrA and KtrB. Contains four repeated domains, each composed of two transmembrane helices connected by a putative pore loop (p-loop). Four conserved glycine residues in the p-loops are part of a selectivity filter for K(+) ions. KtrB alone mediates slow Na(+)-independent K(+) uptake, as well as K(+)-independent Na(+) uptake. Belongs to the TrkH potassium transport family. Ktr (TC 2.A.38.4) subfamily.