ADP-GLUCOSE DRIVES STARCH SYNTHESIS IN ISOLATED MAIZE ENDOSPERM AMYLOPLASTS - CHARACTERIZATION OF STARCH SYNTHESIS AND TRANSPORT-PROPERTIESACROSS THE AMYLOPLAST ENVELOPE

We recently developed a method of purifying amyloplasts from developin g maize (Zen mays L.) endosperm tissue [Neuhaus, Them, Batz and Scheib e (1993) Biochem. J. 296, 395-401]. In the present paper we analyse ho w glucose 6-phosphate (Glc6P) and other phosphorylated compounds enter the plastid compartment. Using a proteoliposome system in which the p lastid envelope membrane proteins are functionally reconstituted, we d emonstrate that this type of plastid is able to transport [C-14]Glc6P or [P-32]P-i in counter exchange with P-i, Glc6P, dihydroxyacetone pho sphate and phosphoenolpyruvate. Glucose 1-phosphate, fructose 6-phosph ate and ribose 5-phosphate do not act as substrates for counter exchan ge. Besides hexose phosphates, ADP-glucose (ADPGlc) also acts as a sub strate for starch synthesis in isolated maize endosperm amyloplasts. T his process exhibits saturation kinetics with increasing concentration s of exogenously supplied [C-14]ADPGlc, reaching a maximum at 2 mM. Ul trasonication of isolated amyloplasts greatly reduces the rate of ADPG lc-dependent starch synthesis, indicating that the process is dependen t on the intactness of the organelles. The plastid ATP/ADP transporter is not responsible for ADPGlc uptake. Data are presented that indicat e that ADPGlc is transported by another translocator in counter exchan ge with AMP. To analyse the physiology of starch synthesis in more det ail, we examined how Glc6P- and ADPGlc-dependent starch synthesis in i solated maize endosperm amyloplasts interact. Glc6P-dependent starch s ynthesis is not inhibited by increasing concentrations of ADPGlc. In c ontrast, the rate of ADPGlc-dependent starch synthesis is reduced by i ncreasing concentrations of ATP necessary for Glc6P-dependent starch s ynthesis. The possible modes of inhibition of ADPGlc-dependent starch synthesis by ATP are discussed with respect to the stromal generation of AMP required for ADPGlc uptake.