Studies on the Entry of Fructose-2,6-Bisphosphate into Chloroplasts

The regulatory metabolite fructose-2,6-bisphosphate (Fru-2,6-P2) has an important function in controlling the intermediary carbon metabolism of leaves. Fru-2,6-P2 controls two cytosolic enzymes involved in the interconversion of fructose-6-phosphate and fructose-1,6-bisphosphate (fructose-1,6-bisphosphatase and pyrophosphate,fructose-6-phosphate 1-phosphotransferase) and thereby controls the partitioning of photosynthate between sucrose and starch. It has been demonstrated that Fru-2,6-P2 is present mainly in the cytosol. Here we present evidence that Fru-2,6-P2 can be taken up by isolated intact chloroplasts but at a very slow rate (about 0.01 micromoles per milligram of chlorophyll per hour). This uptake is time and concentration dependent and is inhibited by PPi. When provided a physiological concentration of Fru-2,6-P2 (10 micromolar), chloroplasts accumulated up to 0.6 micromolar Fru-2,6-P2 in the stroma. Elevated plastid Fru-2,6-P2 levels had no effect on overall photosynthetic rates of isolated chloroplasts. The results indicate that, while Fru-2,6-P2 enters isolated chloroplasts at a sluggish rate, caution should be exercised in ascribing physiological importance to effects of Fru-2,6-P2 on chloroplast enzymes.