title: Switching the clientele: a lysine residing in the C terminus of the serotonin transporter specifies its preference for the coat protein complex II component SEC24C.
abstract: The serotonin transporter (SERT) maintains serotonergic neurotransmission via rapid reuptake of serotonin from the synaptic cleft. SERT relies exclusively on the coat protein complex II component SEC24C for endoplasmic reticulum (ER) export. The closely related transporters for noradrenaline and dopamine depend on SEC24D. Here, we show that discrimination between SEC24C and SEC24D is specified by the residue at position +2 downstream from the ER export motif ((607)RI(608) in SERT). Substituting Lys(610) with tyrosine, the corresponding residue found in the noradrenaline and dopamine transporters, switched the SEC24 isoform preference: SERT-K610Y relied solely on SEC24D to reach the cell surface. This analysis was extended to other SLC6 (solute carrier 6) transporter family members: siRNA-dependent depletion of SEC24C, but not of SEC24D, reduced surface levels of the glycine transporter-1a, the betaine/GABA transporter and the GABA transporter-4. Experiments with dominant negative versions of SEC24C and SEC24D recapitulated these findings. We also verified that the presence of two ER export motifs (in concatemers of SERT and GABA transporter-1) supported recruitment of both SEC24C and SEC24D. To the best of our knowledge, this is the first report to document a change in SEC24 specificity by mutation of a single residue in the client protein. Our observations allowed for deducing a rule for SLC6 family members: a hydrophobic residue (Tyr or Val) in the +2 position specifies interaction with SEC24D, and a hydrophilic residue (Lys, Asn, or Gln) recruits SEC24C. Variations in SEC24C are linked to neuropsychiatric disorders. The present findings provide a mechanistic explanation. Variations in SEC24C may translate into distinct surface levels of neurotransmitter transporters.
authors.affiliation: Institute of Pharmacology, Center of Physiology and Pharmacology, Medical University of Vienna, A-1090 Vienna, Austria.
authors.forename: Sonja
authors.initials: S
authors.lastname: Sucic
authors.forename: Florian
authors.initials: F
authors.lastname: Koban
authors.forename: Ali
authors.initials: A
authors.lastname: El-Kasaby
authors.forename: Oliver
authors.initials: O
authors.lastname: Kudlacek
authors.forename: Thomas
authors.initials: T
authors.lastname: Stockner
authors.forename: Harald H
authors.initials: HH
authors.lastname: Sitte
authors.forename: Michael
authors.initials: M
authors.lastname: Freissmuth
mesh_terms: D020816:Amino Acid Motifs; D001692:Biological Transport; D019921:Biotinylation; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005786:Gene Expression Regulation; D014644:Genetic Variation; D057809:HEK293 Cells; D006367:HeLa Cells; D006801:Humans; D008239:Lysine / Q000737:chemistry*; D016296:Mutagenesis; D009154:Mutation; D018377:Neurotransmitter Agents / Q000378:metabolism; D017434:Protein Structure, Tertiary; D012701:Serotonin / Q000378:metabolism; D050486:Serotonin Plasma Membrane Transport Proteins / Q000235:genetics* / Q000378:metabolism; D014162:Transfection; D050493:Vesicular Monoamine Transport Proteins / Q000378:metabolism; D033921:Vesicular Transport Proteins / Q000235:genetics* / Q000502:physiology*
chemical_list: D018377:Neurotransmitter Agents; C530640:SEC24C protein, human; D050486:Serotonin Plasma Membrane Transport Proteins; D050493:Vesicular Monoamine Transport Proteins; D033921:Vesicular Transport Proteins; D012701:Serotonin; D008239:Lysine