Immature erythroid cells have an exceptionally high capacity to synthesize haem that is, at least in part, the result of the unique control of iron metabolism in these cells. In erythroid cells the vast majority of Fe released from endosomes must cross both the outer and the inner mitochondrial membranes to reach ferrochelatase, which inserts Fe into protoporphyrin IX. Based on the fact that Fe is specifically targeted into erythroid mitochondria, we have proposed that a transient mitochondria-endosome interaction is involved in Fe transfer to ferrochelatase [Ponka (1997) Blood 89, 1–25]. In this study, we examined whether the inhibition of endosome mobility within erythroid cells would decrease the rate of 59Fe incorporation into haem. We found that, in reticulocytes, the myosin light-chain kinase inhibitor, wortmannin, and the calmodulin antagonist, W-7, caused significant inhibition of 59Fe incorporation from 59Fe-transferrin-labelled endosomes into haem. These results, together with confocal microscopy studies using transferrin and mitochondria labelled by distinct fluorescent markers, suggest that, in erythroid cells, endosome mobility, and perhaps their contact with mitochondria, plays an important role in a highly efficient utilization of iron for haem synthesis.

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