Comparison of the kinetic properties, inhibition and labelling of the phosphate translocators from maize and spinach mesophyll chloroplasts.

The kinetic properties of the phosphate translocator from maize (Zea mays L.) mesophyll chloroplasts have been determined. We have used a double silicone-oil-layer centrifugation system in order to obtain true initial uptake rates in forward-reaction experiments. In addition, it was possible to perform back-exchange experiments and to study the effects of illumination and of preloading the chloroplasts with different substrates on transport. It is shown that the phosphate translocator from mesophyll chloroplasts of maize, a C4 plant, transports inorganic phosphate and phosphorylated C3 compounds in which the phosphate group is linked to the C3 atom (e.g. 3-phosphoglycerate and triose phosphate). The affinities of the transported metabolites towards the translocator protein are about one order of magnitude higher than in mesophyll chloroplasts from the C3 plant, spinach. In contrast to the phosphate translocator from C3-mesophyll chloroplasts, that of C4-mesophyll chloroplasts catalyzes in addition the transport of C3 compounds where the phosphate group is attached to the C2 atom (e.g. 2-phosphoglycerate, phosphoenolpyruvate). The phosphate translocator from both chloroplast types is strongly inhibited by pyridoxal-5'-phosphate (PLP), 2,4,6-trinitrobenzenesulfonic acid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). In the case of the spinach translocator protein these inhibitors were shown to react with the same amino-acid residue at the substrate binding site, and one molecule of either DIDS or PLP is obviously required per substrate binding site for the inactivation of the translocation process. In the functionally active dimeric translocator protein only one substrate-binding site appears to be accessible at a particular time, indicating that the site might be exposed to each side of the membrane in turn. Using [(3)H]-H2DIDS for the labelling of maize mesophyll envelopes the radioactivity was found to be associated with two polypeptides of about 29 and 30 kDa. Since Western-blot analysis showed that only the 30 kDa polypeptide reacted with an antiserum directed against the spinach phosphate translocator protein it is suggested that this polypeptide presumably represents the phosphate translocator from maize mesophyll chloroplasts.