title: POSH stimulates the ubiquitination and the clathrin-independent endocytosis of ROMK1 channels.
abstract: POSH (plenty of SH3) is a scaffold protein that has been shown to act as an E3 ubiquitin ligase. Here we report that POSH stimulates the ubiquitination of Kir1.1 (ROMK) and enhances the internalization of this potassium channel. Immunostaining reveals the expression of POSH in the renal cortical collecting duct. Immunoprecipitation of renal tissue lysate with ROMK antibody and glutathione S-transferase pulldown experiments demonstrated the association between ROMK and POSH. Moreover, immunoprecipitation of lysates of HEK293T cells transfected with ROMK1 or with constructs encoding the ROMK-N terminus or ROMK1-C-Terminus demonstrated that POSH binds to ROMK1 on its N terminus. To study the effect of POSH on ROMK1 channels, we measured potassium currents with electrophysiological methods in HEK293T cells and in oocytes transfected or injected with ROMK1 and POSH. POSH decreased potassium currents, and the inhibitory effect of POSH on ROMK channels was dose-dependent. Biotinylation assay further showed that POSH decreased surface expression of ROMK channels in HEK293T cells transfected with ROMK1 and POSH. The effect of POSH on ROMK1 channels was specific because POSH did not inhibit sodium current in oocytes injected with ENaC-alpha, beta, and gamma subunits. Moreover, POSH still decreased the potassium current in oocytes injected with a ROMK1 mutant (R1Delta373-378), in which a clathrin-dependent tyrosine-based internalization signal residing between amino acid residues 373 and 378 is deleted. However, the inhibitory effect of POSH on ROMK channels was absent in cells expressing with dominant negative dynamin and POSHDeltaRING, in which the RING domain was deleted. Expression of POSH also increased the ubiquitination of ROMK1, whereas expression of POSHDeltaRING diminished its ubiquitination in HEK293T cells. The notion that POSH may serve as an E3 ubiquitin ligase is also supported by in vitro ubiquitination assays in which adding POSH increased the ROMK ubiquitination. We conclude that POSH inhibits ROMK channels by enhancing dynamin-dependent and clathrin-independent endocytosis and by stimulating ubiquitination of ROMK channels.
authors.affiliation: Department of Pharmacology, New York Medical College, Valhalla, New York 10595, USA.
authors.forename: Dao-Hong
authors.initials: DH
authors.lastname: Lin
authors.forename: Peng
authors.initials: P
authors.lastname: Yue
authors.forename: Chu-Yang
authors.initials: CY
authors.lastname: Pan
authors.forename: Peng
authors.initials: P
authors.lastname: Sun
authors.forename: Xin
authors.initials: X
authors.lastname: Zhang
authors.forename: Zeguang
authors.initials: Z
authors.lastname: Han
authors.forename: Marcel
authors.initials: M
authors.lastname: Roos
authors.forename: Michael
authors.initials: M
authors.lastname: Caplan
authors.forename: Gerhard
authors.initials: G
authors.lastname: Giebisch
authors.forename: Wen-Hui
authors.initials: WH
authors.lastname: Wang
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000096:biosynthesis* / Q000235:genetics; D000818:Animals; D001692:Biological Transport / Q000502:physiology; D002460:Cell Line; D002966:Clathrin / Q000235:genetics / Q000378:metabolism; D034281:Dynamins / Q000235:genetics / Q000378:metabolism; D053503:Epithelial Sodium Channels / Q000096:biosynthesis / Q000235:genetics; D005786:Gene Expression Regulation / Q000502:physiology*; D006801:Humans; D007685:Kidney Tubules, Collecting; D009865:Oocytes / Q000166:cytology / Q000378:metabolism; D024661:Potassium Channels, Inwardly Rectifying / Q000235:genetics / Q000378:metabolism*; D021382:Protein Sorting Signals / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D044767:Ubiquitin-Protein Ligases / Q000096:biosynthesis* / Q000235:genetics; D054875:Ubiquitination / Q000502:physiology*; D014982:Xenopus laevis
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D002966:Clathrin; D053503:Epithelial Sodium Channels; C086989:KCNJ1 protein, human; C510692:Kcnj1 protein, rat; D024661:Potassium Channels, Inwardly Rectifying; D021382:Protein Sorting Signals; C506024:Sh3rf1 protein, rat; C498193:SH3RF1 protein, human; D044767:Ubiquitin-Protein Ligases; D034281:Dynamins