A number of disorders due to abnormalities in heme biosynthesis influence erythropoiesis. During normal erythroid maturation, heme precursors are generated in the mitochondria, modified in the cytosol and then returned to the mitochondria for final assembly with iron. Remarkably, accumulation of toxic heme intermediates or substrates, and secondary mitochondrial damage, are key elements in the pathophysiology of all heme biosynthesis disorders. However, the mechanisms of export or import of heme products, by-products and intermediates to or from the mitochondrial matrix are as yet poorly understood. This proposal will focus on such mechanisms taking advantage of our discovery of ABC-me, a novel mitochondrial erythroid transporter. ABC-me expression is controlled by the erythroid transcription factor GATA-1 and is down regulated by heme. In differentiating erythroleukemic cells, ABC-me is rate limiting for heme biosynthesis. As such, ABC-me is the only mitochondrial inner membrane transporter implicated in heme biosynthesis. To explore its function in heme biosynthesis, we have developed ABC-me deficient cell culture models and functional assays of reconstituted transporter in proteoliposomes. We have established that differentiating erythroid cells with reduced ABC-me activity produce less heme and exhibit signs of mitochondrial stress. We postulate that transporters involved in the heme pathway serve as gatekeepers for reactive substrates, intermediates and byproducts and thus couple production of essential end products to protection from toxic intermediates. We propose a combined biochemical and biophysical approach to the study of the function of ABC-me in heme biosynthesis and the consequences of its malfunction. We will address the following questions: 1) Which steps in the heme biosynthetic pathway are facilitated by ABC-me? 2) Does ABC-me deficiency result in the accumulation of heme precursors in the cytosol and mitochondria? 3) What is the source of mitochondrial stress associated with ABC-me deficiency and what are the morphological and functional consequences? 4) Does ABC-me transport a heme biosynthesis intermediate, product or co-factor, and in which direction? 5) Does ABC-me facilitate heme biosynthesis indirectly by opposing mitochondrial stress.
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