title: The serotonin transporter is an exclusive client of the coat protein complex II (COPII) component SEC24C.
abstract: The transporters for serotonin (SERT), dopamine, and noradrenaline have a conserved hydrophobic core but divergent N and C termini. The C terminus harbors the binding site for the coat protein complex II (COPII) cargo-binding protein SEC24. Here we explored which SEC24 isoform was required for export of SERT from the endoplasmic reticulum (ER). Three lines of evidence argue that SERT can only exit the ER by recruiting SEC24C: (i) Mass spectrometry showed that a peptide corresponding to the C terminus of SERT recruited SEC24C-containing COPII complexes from mouse brain lysates. (ii) Depletion of individual SEC24 isoforms by siRNAs revealed that SERT was trapped in the ER only if SEC24C was down-regulated, in both, cells that expressed SERT endogenously or after transfection. The combination of all siRNAs was not more effective than that directed against SEC24C. A SERT mutant in which the SEC24C-binding motif ((607)RI(608)) was replaced by alanine was insensitive to down-regulation of SEC24C levels. (iii) Overexpression of a SEC24C variant with a mutation in the candidate cargo-binding motif (SEC24C-D796V/D797N) but not of the corresponding mutant SEC24D-D733V/D734N reduced SERT surface levels. In contrast, noradrenaline and dopamine transporters and the more distantly related GABA transporter 1 relied on SEC24D for ER export. These observations demonstrate that closely related transporters are exclusive client cargo proteins for different SEC24 isoforms. The short promoter polymorphism results in reduced SERT cell surface levels and renders affected individuals more susceptible to depression. By inference, variations in the Sec24C gene may also affect SERT cell surface levels and thus be linked to mood disorders.
authors: Institute of Pharmacology, Center of Physiology and Pharmacology, Medical University of Vienna, Vienna, Austria.
authors: Sonja
authors: S
authors: Sucic
authors: Ali
authors: A
authors: El-Kasaby
authors: Oliver
authors: O
authors: Kudlacek
authors: Subhodeep
authors: S
authors: Sarker
authors: Harald H
authors: HH
authors: Sitte
authors: Philippe
authors: P
authors: Marin
authors: Michael
authors: M
authors: Freissmuth
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D001921:Brain / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism*; D057809:HEK293 Cells; D006367:HeLa Cells; D006801:Humans; D008297:Male; D051379:Mice; D019964:Mood Disorders / Q000235:genetics / Q000378:metabolism; D046912:Multiprotein Complexes / Q000235:genetics / Q000378:metabolism; D020125:Mutation, Missense; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D050486:Serotonin Plasma Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D033921:Vesicular Transport Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D046912:Multiprotein Complexes; D020033:Protein Isoforms; C530640:SEC24C protein, human; C493230:SLC6A4 protein, human; D050486:Serotonin Plasma Membrane Transport Proteins; C493231:Slc6a4 protein, mouse; D033921:Vesicular Transport Proteins