The KdpD protein is a K(+) sensor kinase located in the cytoplasmic membrane of Escherichia coli. It contains four transmembrane stretches and two short periplasmic loops of 4 and 10 amino acid residues, respectively. To determine which part of KdpD functions as a K(+) sensor, genetic variants were constructed with truncations or altered arrangements of the transmembrane segments. All KdpD constructs were tested by complementation of an E. coli kdpD deletion strain for their ability to grow at a K(+) concentration of 0.1 mM in the medium. A soluble protein composed of the C-terminal cytoplasmic domain was able to complement the kdpD deletion strain. In addition, analysis of the beta-galactosidase activity of an E. coli strain which carries a transcriptional fusion of the upstream region of the kdpFABC operon and a promoterless lacZ gene revealed that this soluble KdpD mutant responds to changes in the K(+) concentration in the extracellular medium. The results suggest that the sensing and response functions are both located in the C-terminal domain and might be modulated by the N-terminal domain as well as by membrane anchoring.