Molecular localisation of ferrochelatase in higher plant chloroplasts

Within the chloroplast of higher plants, a crucial branchpoint of the tetrapyrrole synthesis pathway is the chelation of either Fe2+ to make haem, or Mg2+ for chlorophyll, catalysed by ferrochelatase or magnesium chelatase, respectively. One model that has been proposed for the control of this branchpoint, based on biochemical studies, is that the two enzymes are spatially separated within the chloroplast, ferrochelatase being exclusively in the thylakoids, while magnesium chelatase is associated with the envelope [Matringe, M., Camadro, J.-M., Joyard, J. & Douce, R. (1994) J. Biol. Chem. 269, 15010-15015]. We have used a sensitive molecular method to investigate this possibility. Radiolabelled precursor proteins for ferrochelatase from Arabidopsis have been imported into isolated chloroplasts. Their distribution in the different subchloroplastic fractions have then been determined, and compared with that for light-harvesting chlorophyll protein, which is exclusively thylakoidal, and the envelope-located phosphate translocator. Clear evidence for the specific association of ferrochelatase protein with both thylakoid and envelope membranes has been obtained, thus suggesting strongly that the control of the branchpoint cannot be by spatial separation of the two chelatases.