The tricarboxylate transporter was solubilized from pea (Pisum sativum) mitochondria with Triton X-1 14, partially purified over a hydroxylapatite column, and reconstituted in phospholipid vesicles.The proteoliposomes exchanged external [14C] (7,9,17,30).Although mitochondrial transporters are found in both plants and animals, there are some differences in the types of transporters present. Plant leaf mitochondria, for example, rapidly exchange glycine and L-senne to support the photorespiratory conversion of glycine to seine (19,27 for the phosphate transporter has been obtained from Arabidopsis (our unpublished data). Several organic acid transporters have been solubilized from mitoplasts, partially purified, and incorporated into liposomes. The monocarboxylate, dicarboxylate (24), glutamate/aspartate (25), and aketoglutarate (10) transporters have been isolated from plant mitochondria and incorporated into proteoliposomes for detailed study. However, sequence information for the organic acid transporters is not yet available from either plants or animals.The tricarboxylate transporter from animal mitochondria has been studied in considerable detail (6,16,(21)(22)(23). This transporter from rat (4) and bovine (6) mitochondria has been isolated and purified 1000-and 400-fold, respectively. The preparation from rat liver showed a protein with an apparent molecular mass of 30 kD, whereas the preparation from bovine liver had a predominant 37-to 38-kD protein on SDS-PAGE gels. No conclusive proof is available that either band is the mammalian tricarboxylate transporter. Both transporters exchanged citrate, isocitrate, malate, malonate, and phosphoenolpyruvate. In this paper, we report the solubilization, partial purification, reconstitution, and analysis of the tricarboxylate transporter from pea (Pisum sativum L.) mitochondria. buffer B containing 6 mg of cardiolipin mL-1 was added and the sample was incubated for 30 min. Solubilized membranes were then centrifuged for 35 min at 37,000g and the supernatant was removed. This crude Triton X-1 14 extract was then chromatographed on cold dry hydroxylapatite (0.5 g per column, 0.5 mL of crude extract per column). Elution was done with buffer B and the first 1.0 mL off each column was collected and pooled.
MATERIALS AND METHODS
Preparation of ProteoliposomesDried azolectin was resuspended in buffer C (120 mm Hepes, 50 mm KCl, 1 mm EDTA, pH 7.5) under N2 gas at 120 mg/mL and bath sonicated (Laboratory Supplies Co.) at room temperature until the solution was translucent. Six microliters of a 1-M solution of the organic to be loaded into the proteoliposome (citrate, malate, etc.) was added to 540 ,uL of liposomes and then 460 ,L (6.5 ,ug of protein) of the hydroxylapatite eluate (or 200 ML of the crude Triton X-114 extract containing about 4 mg of protein and 260 ,uL of buffer B) was added. The sample was vortexed and frozen at -800C.Immediately prior to the assay, the proteoliposomes were thawed for 15 min in a room-temperature water bath, cooled on ice for 5 mi...