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 (P1) or glucose 6-phosphate (Glc6P). In both cases, the rate of import is strongly dependent upon preloading of proteoliposomes with either P1, dihydroxyacetone phosphate (DHAP) or Glc6P. This demonstrates the presence of a phosphate translocator activity catalysing a counter exchange of phosphorylated intermediates. Interestingly, a high external concentration of Glc6P does not strongly inhibit P1 uptake into proteoliposomes preloaded with DHAP, whereas external Glc6P strongly inhibits P1 uptake into proteoliposomes preloaded with Glc6P. 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.