title: Serotonin transamidates Rab4 and facilitates its binding to the C terminus of serotonin transporter.
abstract: The serotonin transporter (SERT) on the plasma membrane is the major mechanism for the clearance of plasma serotonin (5-hydroxytryptamine (5HT)). The uptake rates of cells depend on the density of SERT molecules on the plasma membrane. Interestingly, the number of SERT molecules on the platelet surface is down-regulated when plasma 5HT ([5HT](ex)) is elevated. It is well reported that stimulation of cells with high [5HT](ex) induces transamidation of a small GTPase, Rab4. Modification with 5HT stabilizes Rab4 in its active, GTP-bound form, Rab4-GTP. Although investigating the mechanism by which elevated plasma 5HT level down-regulates the density of SERT molecules on the plasma membrane, we studied Rab4 and SERT in heterologous and platelet expression systems. Our data demonstrate that, in response to elevated [5HT](ex), Rab4-GTP co-localizes with and binds to SERT. The association of SERT with Rab4-GTP depends on: (i) 5HT modification and (ii) the GTP-binding ability of Rab4. Their association retains transporter molecules intracellularly. Furthermore, we mapped the Rab4-SERT association domain to amino acids 616-624 in the cytoplasmic tail of SERT. This finding provides an explanation for the role of the C terminus in the localization and trafficking of SERT via Rab4 in a plasma 5HT-dependent manner. Therefore, we propose that elevated [5HT](ex)"paralyzes" the translocation of SERT from intracellular locations to the plasma membrane by controlling transamidation and Rab4-GTP formation.
authors.affiliation: Department of Biochemistry and Molecular Biology, College of Medicine, University of Arkansas for Medical Sciences, 301 West Markham Street, Little Rock, AR 72205, USA.
authors.forename: Billow A
authors.initials: BA
authors.lastname: Ahmed
authors.forename: Brandon C
authors.initials: BC
authors.lastname: Jeffus
authors.forename: Syed I A
authors.initials: SI
authors.lastname: Bukhari
authors.forename: Justin T
authors.initials: JT
authors.lastname: Harney
authors.forename: Resat
authors.initials: R
authors.lastname: Unal
authors.forename: Vladimir V
authors.initials: VV
authors.lastname: Lupashin
authors.forename: Peter
authors.initials: P
authors.lastname: van der Sluijs
authors.forename: Fusun
authors.initials: F
authors.lastname: Kilic
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000166:cytology / Q000378:metabolism*; D016466:CHO Cells; D002462:Cell Membrane / Q000378:metabolism*; D006224:Cricetinae; D003412:Cricetulus; D003593:Cytoplasm / Q000378:metabolism; D015536:Down-Regulation / Q000187:drug effects / Q000502:physiology; D006160:Guanosine Triphosphate / Q000235:genetics; D006367:HeLa Cells; D006801:Humans; D011499:Protein Processing, Post-Translational / Q000187:drug effects* / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology; D021381:Protein Transport / Q000502:physiology; D012701:Serotonin / Q000378:metabolism / Q000494:pharmacology*; D050486:Serotonin Plasma Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D020695:rab4 GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D050486:Serotonin Plasma Membrane Transport Proteins; D012701:Serotonin; D006160:Guanosine Triphosphate; D020695:rab4 GTP-Binding Proteins