A reversible carrier mediates the transport of malate at the tonoplast of Catharanthus roseus cells.

The conditions of malate transport were defined in tonoplast vesicles purified from a microsomal homogenate of Catharanthus roseus cells by preparative free-flow electrophoresis. Isolated vesicles exhibited malate transport when the membranes were prepared by grinding the cells in a homogenisation medium only buffered in the acidic pH range. By using vesicles energized artificially by an imposed pH gradient (acid interior), it was shown that malate is actively accumulated in response to the generation of a proton-motive force. Several lines of evidence (saturation kinetics, action of malate analogs and protein modifiers) support the concept that malate transport is mediated by a protein carrier which could be implicated in the uptake process as its protonated form. The malate transported in the vesicles was released by lowering the external malate concentration. The release was prevented by the anion transport inhibitor DIDS indicating the reversibility of the carrier.