Mutations in any one of three genes, kdpA, -B, or -C, in Escherichia coli abolish the activity of Kdp, a multisubunit K⁺-ATPase that belongs to the P-type ATPase family of cation transporters. We found in this study that expression in vivo of a 135-amino-acid-long N-terminal fragment (KdpA'), less than one-quarter the length of native KdpA, was able to mediate an improvement in K⁺-limited growth rates in two different contexts, even in the absence of both KdpC and the ATPase subunit KdpB. The first context was when KdpA' was overexpressed in cells from a heterologous inducible promoter, and the second was when KdpA' was provided with a C-terminally altered extension (following a spontaneous genetic rearrangement). Our results suggest that KdpA' provides an incipient pathway for K⁺ translocation which can serve to transport K⁺ into the cells in response to the cytoplasmic membrane potential.