In insect epithelia, the transport of alkali-metal cations (Na+ and K+) across the apical plasma membrane is thought to be driven by two transmembrane ion transporters that act in concert – a proton-motive V-ATPase isoform and a cation-proton exchanger. The identity of the putative exchanger has been elusive, but Julian Dow and colleagues () now show that the Drosophila proteins CG10806 and CG31052 – which are closely related to the bacterial Kef exchanger family – are candidates for this role. Using FlyAtlas microarray data and in situ hybridisation, the authors show that – similar to the V-ATPase – both CG10806 and CG31052 are expressed in epithelial cells of the Drosophila hindgut and Malpighian (renal) tubule; moreover, both proteins localise to the apical plasma membrane. The overexpression of CG10806 increases the basal level of V-ATPase-dependent fluid secretion in renal tubules, and CG31052 overexpression increases both the Na+ and K+ concentration in secreted fluid. In exchanger-deficient yeast, CG10806 expression confers protection against excess K+, whereas CG31052 protects against excess Na+. Together, these results indicate that CG10806 and CG31052 might act as K+-proton and Na+-proton exchangers, respectively, in Drosophila epithelia.
