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Evidence for a Homodimeric Structure of Human Monocarboxylate Transporter 8

Departments of Internal Medicine (W.E.V., W.K., E.C.H.F., J.J., T.J.V.) and Cell Biology (T.B.v.D.), Erasmus University Medical Center, 3015 GE Rotterdam, The Netherlands; Department of Pathology, Anatomy, and Cell Biology (N.J.P.), Thomas Jefferson University, Philadelphia, Pennsylvania 19107; School of Biological Sciences (P.W.B.), Royal Holloway University of London, London E1 4NS, United Kingdom; and Department of Biopharmaceutical Sciences (A.G.I.), University of California, San Francisco, San Francisco, California 94143

Address all correspondence and requests for reprints to: Theo J. Visser, Erasmus University Medical Center, Dr Molewaterplein 50, 3015 GE Rotterdam, The Netherlands. E-mail: t.j.visser{at}erasmusmc.nl.

The human monocarboxylate transporter 8 (hMCT8) protein mediates transport of thyroid hormone across the plasma membrane. Association of hMCT8 mutations with severe psychomotor retardation and disturbed thyroid hormone levels has established its physiological relevance, but little is still known about the basic properties of hMCT8. In this study we present evidence that hMCT8 does not form heterodimers with the ancillary proteins basigin, embigin, or neuroplastin, unlike other MCTs. In contrast, it is suggested that MCT8 exists as monomer and homodimer in transiently and stably transfected cells. Apparently hMCT8 forms stable dimers because the complex is resistant to denaturing conditions and dithiothreitol. Cotransfection of wild-type hMCT8 with a mutant lacking amino acids 267–360 resulted in formation of homo-and heterodimers of the variants, indicating that transmembrane domains 4–6 are not involved in the dimerization process. Furthermore, we explored the structural and functional role of the 10 Cys residues in hMCT8. All possible Cys>Ala mutants did not behave differently from wild-type hMCT8 in protein expression, cross-linking experiments with HgCl₂ and transport function. Our findings indicate that individual Cys residues are not important for the function of hMCT8 or suggest that hMCT8 has other yet-undiscovered functions in which cysteines play an essential role.