Functional properties of multiple isoforms of human divalent metal-ion transporter 1 (DMT1)

Biochemical Journal - Tập 403 Số 1 - Trang 59-69 - 2007
Bryan Mackenzie1,2, Hitomi Takanaga2, Nadia Hubert3, Andreas Rolfs2, Matthias A. Hediger2
1Department of Molecular and Cellular Physiology, University of Cincinnati College of Medicine, PO Box 670576, Cincinnati, OH 45267-0576, U.S.A.
2Membrane Biology Program and Renal Division, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, 77 Avenue Louis Pasteur, Boston, MA 02115, U.S.A.
3European Molecular Biology Laboratory Gene Expression Programme Meyerhofstrasse 1 D-69117 Heidelberg, Federal Republic of Germany

Tóm tắt

DMT1 (divalent metal-ion transporter 1) is a widely expressed metal-ion transporter that is vital for intestinal iron absorption and iron utilization by most cell types throughout the body, including erythroid precursors. Mutations in DMT1 cause severe microcytic anaemia in animal models. Four DMT1 isoforms that differ in their N- and C-termini arise from mRNA transcripts that vary both at their 5′-ends (starting in exon 1A or exon 1B) and at their 3′-ends giving rise to mRNAs containing (+) or lacking (−) the 3′-IRE (iron-responsive element) and resulting in altered C-terminal coding sequences. To determine whether these variations result in functional differences between isoforms, we explored the functional properties of each isoform using the voltage clamp and radiotracer assays in cRNA-injected Xenopus oocytes. 1A/IRE(+)-DMT1 mediated Fe2+-evoked currents that were saturable (K0.5Fe≈1–2 μM), temperature-dependent (Q10≈2), H+-dependent (K0.5H≈1 μM) and voltage-dependent. 1A/IRE(+)-DMT1 exhibited the provisional substrate profile (ranked on currents) Cd2+, Co2+, Fe2+, Mn2+>Ni2+, V3+≫Pb2+. Zn2+ also evoked large currents; however, the zinc-evoked current was accounted for by H+ and Cl− conductances and was not associated with significant Zn2+ transport. 1B/IRE(+)-DMT1 exhibited the same substrate profile, Fe2+ affinity and dependence on the H+ electrochemical gradient. Each isoform mediated 55Fe2+ uptake and Fe2+-evoked currents at low extracellular pH. Whereas iron transport activity varied markedly between the four isoforms, the activity for each correlated with the density of anti-DMT1 immunostaining in the plasma membrane, and the turnover rate of the Fe2+ transport cycle did not differ between isoforms. Therefore all four isoforms of human DMT1 function as metal-ion transporters of equivalent efficiency. Our results reveal that the N- and C-terminal sequence variations among the DMT1 isoforms do not alter DMT1 functional properties. We therefore propose that these variations serve as tissue-specific signals or cues to direct DMT1 to the appropriate subcellular compartments (e.g. in erythroid cells) or the plasma membrane (e.g. in intestine).

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Biochemical Journal

Tập 403 Số 1

59-69

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