Evidence for two types of phosphate translocators in sweet-pepper (Capsicum annum L.) fruit chromoplasts

We have investigated whether there is evidence for the presence of different types of phosphate translocators in envelopes purified from pepper-fruit chromoplasts. A method was developed that allowed the purification of envelope membranes from isolated pepper-fruit chromoplasts. Proteoliposomes containing envelope-membrane proteins are able to import inorganic phosphate (P i ) or glucose 6-phosphate (Glc6 P ). In both cases, the rate of import is strongly dependent upon preloading of proteoliposomes with either P i , dihydroxyacetone phosphate (DHAP) or Glc6 P . This demonstrates the presence of a phosphate translocator activity catalysing a counter exchange of phosphorylated intermediates. Interestingly, a high external concentration of Glc6 P does not strongly inhibit P i uptake into proteoliposomes preloaded with DHAP, whereas external Glc6 P strongly inhibits P i uptake into proteoliposomes preloaded with Glc6 P . This observation strongly indicates that two types of phosphate translocator are present in chromoplast envelopes from red-pepper fruits. These data are discussed with respect to the possible physiological function of two types of phosphate translocator in one type of plastid.