ANALYSIS OF CARBOHYDRATE TRANSPORT ACROSS THE ENVELOPE OF ISOLATED CAULIFLOWER-BUD AMYLOPLASTS

Using isolated amyloplasts from cauliflower buds, we have characterize d the interaction and transport of various carbohydrates across the en velope membrane of a heterotrophic plastid. According to our results, glucose 6-phosphate (Glc6P) and glucose 1-phosphate (Glc1 P) do not sh are the same transport protein for uptake into cauliflower-bud amylopl asts. Glc6P-dependent starch synthesis is strongly inhibited in the pr esence of dihydroxyacetone phosphate (DHAP) or 4,4'-di-isothiocyano-2, 2'-stilbenedisulphonic acid (DIDS), whereas Glc1P-dependent starch syn thesis is hardly affected by these compounds. Analysis of the Glc6P up take into proteoliposomes reconstituted from the envelope proteins of cauliflower-bud amyloplasts indicate that Glc6P is taken up in a count er-exchange mode with P-i, DHAP or Glc6P, whereas Glc1P does not act a s a counter-exchange substrate. P-i is a strong competitive inhibitor of Glc6P uptake (K-i 0.8 mM) into proteoliposomes, whereas Glc1P does not significantly inhibit Glc6P transport. Beside a hexose-phosphate t ranslocator, these amyloplasts possess an envelope protein mediating t he transport of glucose across the membrane. This translocator exhibit s an apparent K-m for glucose of 2.2 mM and is inhibited by low concen trations of phloretin, known to be a specific inhibitor of glucose-tra nsport proteins. Maltose inhibits the uptake of glucose (K-i 2.3 mM), indicating that both carbohydrates share the same translocator.