Evidence for a glucose 1‐phosphate translocator in storage tissue amyloplasts of potato (Solanum tuberosum) suspension‐cultured cells

Abstract

Transport of glucose 1‐phosphate (G1P) and highly purified triose phosphate into storage tissue amyloplasts was studied. Isolated amyloplasts from potato (Solanum tuberosum L., dihaploid stock, HH 258) were transport‐functional and metabolically active in starch synthesis. Fourty percent of the amyloplasts were intact and there was only a small degree (0–1.6%) of contamination by other cellular compartments. G1P showed a clear uptake pattern paralleled by starch synthesis. Uptake of triose phosphates was virtually nil. Uptake of GIP was pH dependent with a sharp maximum at pH 5.7 and showed Michaelis‐Menten kinetics with an apparent Km of 0.5 mM. Temperature influenced the rate of uptake, the highest rate being at 25°C. Fructose l‐phosphate, ADP‐glucose, glucose, and inorganic phosphate inhibited the uptake of G1P. Uptake was also inhibited by DIDS (1–25 μM) and by Phloretin (45–750 μW). It is therefore concluded that the transport of GIP across the inner amyloplast membrane is mediated by a hexose phosphate translocator selective for phosphate and glucose moieties of the molecule. Considering the low pH maximum for G1P uptake it is possible that the uptake of G1P, and eventually starch synthesis, is regulated by an acidification of the intermembrane space by proton pumps of the inner amyloplast membrane.