Purification of two forms of starch branching enzyme (Q-enzyme) from developing rice endosperm

Two isoforms of starch branching enzyme (Q-enzyme), QEI and QEII, have been purified to honlogeneity from developing rice endosperm. QEI and QEII, with molecular weights of about 80 and 85 kDa, respectively, could be fully separated by anion-exchange or hydrophobic chromatography. The peptide maps obtained after V₈ proteinase digestion were quite different between the two enzymes. Antibodies prepared against QEI showed no immunological cross-reaction with the QEII protein in Western blot experiments, and anti-QEII serum did not react with the QEI protein. The data indicate that QEI and QEII are distinct proteins encoded by different genes in rice plants.