A CDPK type protein kinase is involved in rice SPS light modulation

A protein kinase activity that can phosphorylate and inactivate rice (Oryza sativa) sucrose-phosphate synthase (SPS; UDP-glucose: d-fructose-6-phosphate-2-glucosyl transferase, EC 2.4.1.14) was measured in extracts prepared from leaves exposed to light-dark transitions. Enzyme activity present in extracts from dark leaves was about 5-fold higher than the activity in extracts from leaves that had been collected in the light. The protein kinase (named R-SPSK) was purified about 100-fold from dark leaves and its biochemical properties were studied. The micromolar dependence of Ca2+ exhibited by R-SPSK, and its response to calmodulin antagonists was similar to the properties associated with members of the plant Calcium-Dependent Protein Kinase (CDPK) family. Two modulators of SPS activity, Pi and Glc-6-P, were examined for an effect on R-SPSK. While Glc-6-P did not affect R-SPSK activity, Pi drastically increased the kinase activity. Taken together, these data provide evidence that SPS may be regulated by a CDPK type protein-kinase whose activity is modulated by light-dark transitions and stimulated by Pi, the negative effector of SPS activity.