1-aminocyclopropane-1-carboxylate oxidase of apple fruit is periplasmic

Immunocytological studies have previously shown that 1-aminocyclopropane-1-carboxylate oxidase (ACO), the enzyme which catalyses the last step of ethylene biosynthesis, is located in the cell wall of apple and tomato fruit cells. In the present study, a combination of cell fractionation and immunocytological methods have been used in order to determine a precise location within this space. Western blotting assays indicated that more than 70% of ACO antigens of the whole cell are recovered in freshly prepared protoplasts and that these ACO antigens are completely removed upon treatment of protoplasts with proteinase K. Immunocytolabelling showed a periplasmic AGO-antigen signal in protoplasts which is completely absent in proteinase K-treated protoplasts. Taken together, these data demonstrate that, in apple fruit, ACO is located at the external face of the plasma membrane. Possible interactions between the plasma membrane and ACO activity are discussed.