Vacuolar citrate/H⁺ symporter of citrus juice cells

We have isolated a cDNA, designated Citrus sinensis citrate transporter 1 CsCit1 encoding a novel vacuolar citrate/symporter. Immunoblots using antibodies raised against CsCit1 showed that the protein is localized to the juice sac cell vacuoles. The highest expression of CsCit1 and the amount of protein in the juice sac cell vacuoles coincided with the developmental stage at which the vacuolar citrate content began declining with the concomitant increase in vacuolar pH. Vacuoles from Sacharomyces cereviseae expressing CsCit1 displayed a citrate-dependent H⁺ efflux, and our results clearly demonstrate that CsCit1 is able to mediate the electroneutral co-transport of H⁺ and citrate ions, since the citrate-dependent H⁺ fluxes are not affected by changing the electrical potential difference across the tonoplast. The roles of CsCit1 in mediating citrate efflux from the vacuole and on citric acid homoestasis in Citrus juice sac cells are discussed.