Subcellular Localization of Anthocyanin Methyltransferase in Flowers of Petunia hybrida.

The subcellular localization of the enzyme anthocyanin-methyltransferase was studied in cells (protoplasts) obtained from the upper epidermis of petals of Petunia hybrida Hort. Vacuoles were isolated from protoplasts to ascertain the possible presence of the enzyme in these organelles. The recovery of methyltransferase activity in vacuole-enriched fractions equalled that of the cytosolic marker enzyme glucose-6-phosphate dehydrogenase. The relative activity of methyltransferase in the vacuole fraction was one tenth of that in the protoplast. Neither whole protoplasts nor isolated vacuoles contained inhibitors of methyltransferase activity. Examination of fractions obtained by differential centrifugation of a protoplast lysate showed that the major part of the methyltransferase activity was cytosolic. Activity found in a 130,000g pellet was due to nonspecific adhesion to membranes. The results indicate that terminal steps of anthocyanin biosynthesis take place in the cytosol. They do not lend support to the notion that the vacuole might be involved in (part of) this process.