Characterization of ATP-Dependent Fructose 6-Phosphate 1-Phosphotransferase Isozymes from Leaf and Endosperm Tissues of Ricinus communis

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Abstract
Plastid and cytosolic isozymes of ATP:fructose 6-phosphate 1-phosphotransferase (PFKp and PFKc, respectively) have been isolated from leaves and developing endosperm tissues of the castor oil plant (Ricinus communis L). Endosperm PFKp has been purified to apparent homogeneity. Polyclonal antibodies raised against one of the four polypeptides associated with potato tuber PFK (molecular mass, 46 kilodaltons) immunoprecipitated developing endosperm and leaf PFKp, but not PFKc isozymes. Western blots, sodium dodecyl sulfate polyacrylamide gel electrophoresis, and analytical gel filtration show that PFKp from developing endosperm is a 220 kilodalton homotetramer composed of 57 kilodalton subunits. Kinetic studies of leaf PFKp and PFKc isozymes reveal both similarities and differences to the characteristics of the respective endosperm isozymes studied previously (WJ Garland, DT Dennis [1980] Arch Biochem Biophys 204: 302-317). The immunological and kinetic data suggest that leaf and developing endosperm PFKp are different but structurally related proteins.