title: Asn441 plays a key role in folding and function of the Na+/I- symporter (NIS).
abstract: The Na(+)/I(-) symporter (NIS) is a plasma membrane glycoprotein that mediates active I(-) transport in the thyroid, the first 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 a thorough characterization of the ITD-causing NIS mutation in which the sixth intracellular loop residues 439-443 are missing. This mutant protein was intracellularly retained, incompletely glycosylated, and intrinsically inactive. Engineering 5 Ala at positions 439-443 partially recovered cell surface targeting and activity (∼15%). Strikingly, NIS with the sequence 439-AANAA-443, in which Asn was restored at position 441, was targeted to the plasma membrane and exhibited ∼95% the transport activity of WT NIS. Based on our NIS homology model, we propose that the side chain of N441, a residue conserved throughout most of the SLC5 family, interacts with the main chain amino group of G444, capping the α-helix of transmembrane segment XII and thus stabilizing the structure of the molecule. Our data provide insight into a critical interhelical interaction required for NIS folding and activity.
authors: Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York, USA.
authors: Wenjing
authors: W
authors: Li
authors: Juan Pablo
authors: JP
authors: Nicola
authors: L Mario
authors: LM
authors: Amzel
authors: Nancy
authors: N
authors: Carrasco
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001216:Asparagine / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites / Q000235:genetics; D001692:Biological Transport / Q000235:genetics; D019556:COS Cells; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D002522:Chlorocebus aethiops; D006801:Humans; D015151:Immunoblotting; D007455:Iodine / Q000378:metabolism; D018613:Microscopy, Confocal; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D027981:Symporters / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D022161:Transport Vesicles / Q000378:metabolism; D000099209:Sodium-Iodide Symporters
chemical_list: D001216:Asparagine; D007455:Iodine; D027981:Symporters; D000099209:Sodium-Iodide Symporters
keywords: congenital hypothyroidism; helix capping; interhelical interaction; plasma membrane targeting; protein folding
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