Does the KdpA subunit from the high affinity K+-translocating P-type KDP-ATPase have a structure similar to that of K+ channels?

Evidence is presented that the transmembrane KdpA subunit of the high affinity K+-translocating P-type Kdp-ATPase is evolutionarily derived from the superfamily of 2TM-type K+ channels in bacteria. This extends a previous study relating the K+ channels to the KtrAB, Trk, Trk1,2, and HKT1 K+ symporter superfamily of both prokaryotes and eukaryotes, Although the channels are formed by four single-MPM motif subunits, the transmembrane KdpA subunit and the transmembrane subunit of the symporter proteins are postulated to have four corresponding MPM motifs within a single sequence. Analysis of 17 KdpA sequences reveals a pattern of residue conservation similar to that of the symporters and channels, and consistent with the crystal structure of the KcsA K+ channel. In addition, the most highly conserved residues between the families, specifically the central glycines of the P2 segments, are those previously identified as crucial for the property of K+-selectivity that is common to each protein. This hypothesis is consistent with an experimental study of mutations that alter K+ binding affinity of the Kdp transporter. Although most of the results of a previous study of the transmembrane topology of KdpA are consistent with the 4-MPM model, the one deviation can be explained by a plausible change in the structure due to the experimental method.