Adenosine 5'-triphosphate-mediated activation of sucrose-phosphate synthase in bundle sheath cells of C₄ plants

We report the ATP-mediated activation of sucrose-phosphate synthase in bundle sheath cells prepared from C₄ species. Sucrose synthesis was followed by measuring the incorporation of [¹⁴C]fructose 6-phosphate into sucrose in bundle sheath cells also provided with uridine 5'-diphosphoglucose (UDPGlc). Studies with Panicum miliaceum L. cells showed that activation was largely due to an increase in the affinity for UDPGlc and was therefore only evident at limiting UDPGlc concentrations. The apparent K(m) UDPGlc for sucrose synthesis by cells pretreated and assayed with ATP was about 0.7 mM compared with 7-8 mM for control cells without ATP. The γ-thio derivative of ATP had a similar effect to ATP. The effect was also evident when ATP was rapidly removed from cells prior to assay. Sucrose-phosphate synthase activity in extracts from cells pretreated with or without ATP showed similar differences in K(m) UDPGlc. These observations support the view that ATP is inducing a covalent modification of the enzyme. However, several protein kinase inhibitors did not prevent activation. Changes of more than threefold were observed for the K(m) UDPGlc with sucrose-phosphate synthase extracted from bundle sheath cells rapidly isolated from attached leaves that were subjected to dark/light treatments. The possible relationship between these changes and those induced by ATP with isolated cells is discussed.