title: The iodide-transport-defect-causing mutation R124H: a δ-amino group at position 124 is critical for maturation and trafficking of the Na+/I- symporter.
abstract: Na(+)/I(-) symporter (NIS)-mediated active accumulation of I(-) in thyrocytes is a key step in the biosynthesis of the iodine-containing thyroid hormones T3 and T4. Several NIS mutants have been identified as a cause of congenital I(-) transport defect (ITD), and their investigation has yielded valuable mechanistic information on NIS. Here we report novel findings derived from the thorough characterization of the ITD-causing mutation R124H, located in the second intracellular loop (IL-2). R124H NIS is incompletely glycosylated and colocalizes with endoplasmic reticulum (ER)-resident protein markers. As a result, R124H NIS is not targeted to the plasma membrane and therefore does not mediate any I(-) transport in transfected COS-7 cells. Strikingly, however, the mutant is intrinsically active, as revealed by its ability to mediate I(-) transport in membrane vesicles. Of all the amino acid substitutions we carried out at position 124 (K, D, E, A, W, N and Q), only Gln restored targeting of NIS to the plasma membrane and NIS activity, suggesting a key structural role for the δ-amino group of R124 in the transporter's maturation and cell surface targeting. Using our NIS homology model based on the structure of the Vibrio parahaemolyticus Na(+)/galactose symporter, we propose an interaction between the δ-amino group of either R or Q124 and the thiol group of C440, located in IL-6. We conclude that the interaction between IL-2 and IL-6 is critical for the local folding required for NIS maturation and plasma membrane trafficking.
authors.affiliation: Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
authors.forename: Viktoriya
authors.initials: V
authors.lastname: Paroder
authors.forename: Juan P
authors.initials: JP
authors.lastname: Nicola
authors.forename: Christopher S
authors.initials: CS
authors.lastname: Ginter
authors.forename: Nancy
authors.initials: N
authors.lastname: Carrasco
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D002462:Cell Membrane / Q000235:genetics / Q000378:metabolism; D002522:Chlorocebus aethiops; D004285:Dogs; D006801:Humans; D007454:Iodides / Q000378:metabolism*; D061985:Madin Darby Canine Kidney Cells; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D017510:Protein Folding; D021381:Protein Transport; D013329:Structure-Activity Relationship; D027981:Symporters / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D014162:Transfection; D000099209:Sodium-Iodide Symporters
chemical_list: D007454:Iodides; D027981:Symporters; D000099209:Sodium-Iodide Symporters
keywords: Congenital iodide transport defect; Impaired intracellular trafficking; Membrane vesicles; Na+/I- symporter (NIS); Plasma membrane targeting