Malate and malate-channel antibodies inhibit electrogenic and ATP-dependent citrate transport across the tonoplast of citrus juice cells

Citrus juice cells accumulate high levels of citric acid in their vacuoles when compared to other organic ions including malate. Uptake of citrate into tonoplast vesicles from Citrus juice cells was investigated in the presence of malate, and after incubation with antibodies raised against the vacuolar malate-specific channel of Kalanchoë diagremontiana leaves. Antibodies against the vacuolar malate channel immunoreacted with a protein of similar size in tonoplast extracts from three Citrus varieties differing in citric acid content. Malate channel antibodies inhibited both δμH+-dependent and ΔμH+-independent ATP-dependent citrate transport, indicating common domains in both transport systems and to the malate-specific channel of Kalanchoë diagremontiana leaves. Malate strongly inhibited electrogenic citrate transport, whereas ATP-dependent citrate uptake was less affected. Kinetic analysis of citrate transport in the presence of malate confirmed the existence of two citrate transport mechanisms and indicated that both citrate and malate share a common transport channel across the tonoplast of Citrus juice cells.