Regulation of Activity by Protein Level during the Import to Export Transition

The maize (Zea mays) leaf is a valuable system to study the sucrose import to sucrose export transition at the cellular level. Rapidly growing and fully heterotrophic cells in the basal part of the young leaf showed a high sucrose synthase (SS) activity. Leaf SS has been purified to homogeneity. By comparison with purified kernel SS isozymes, the leaf SS has been identified as SS2. SS1 protein and SS2 protein were clearly separated by electrophoresis and the two monomers differed in size by 6 kilodaltons. Nevertheless, kinetic parameters of both enzymes were very similar. Immunodetection of SS protein showed that in young heterotrophic tissues SS2 was a major protein accounting for 3% of the total protein. Concurrent with greening, SS activity decreased and the change of activity was explained by regulation of the protein level. In mature green tissues, which are synthetizing sucrose as evidenced by the presence of sucrose phosphate synthase activity, SS activity was almost completely absent. Results suggested that down regulation of SS2 enzyme protein level was an early event in the transition from import to export status of the leaf.