title: A phosphorylation-dependent export structure in ROMK (Kir 1.1) channel overrides an endoplasmic reticulum localization signal.
abstract: The cell surface density of functional Kir1.1 (ROMK, KCNJ1) channels in the renal collecting duct is precisely regulated to maintain potassium balance. Here, we explore the mechanism by which phosphorylation of Kir1.1a serine 44 controls plasmalemma expression. Studies in Xenopus oocytes, expressing wild-type, phosphorylation mimic (S44D), or phosphorylation null (S44A) Kir1.1a, revealed that phosphorylation of serine 44 is required to stimulate traffic of newly synthesized channels to the plasma membrane through a brefeldin A-sensitive pathway. ROMK channels were found to acquire mature glycosylation in a serine 44 phosphorylation-dependent manner, consistent with a phosphorylation-dependent trafficking step within the endoplasmic reticulum/Golgi. Serine 44 neighbors a string of three "RXR" motifs, reminiscent of basic trafficking signals involved in directing early transport steps within the secretory pathway. Replacement of the arginine residues with alanine (R35A, R37A, R39A, R41A, or all Arg to Ala) did not restore cell surface expression of the phospho-null S44A channel, making it unlikely that phosphorylation abrogates a nearby RXR-type endoplasmic reticulum (ER) localization signal. Instead, analysis of the compound S44D phospho-mimic mutants revealed that the neighboring arginine residues are also necessary for cell surface expression, identifying a structure that determines export in the biosynthetic pathway. Suppressor mutations in a putative dibasic ER retention signal, located within the cytoplasmic C terminus (K370A, R371A), restored cell surface expression of the phospho-null S44A channel to levels exhibited by the phospho-mimic S44D channel. Taken together, these studies indicate that phosphorylation of Ser44 drives an export step within the secretory pathway to override an independent endoplasmic reticulum localization signal.
authors.affiliation: Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA.
authors.forename: Dana
authors.initials: D
authors.lastname: Yoo
authors.forename: Liang
authors.initials: L
authors.lastname: Fang
authors.forename: Amanda
authors.initials: A
authors.lastname: Mason
authors.forename: Bo-Young
authors.initials: BY
authors.lastname: Kim
authors.forename: Paul A
authors.initials: PA
authors.lastname: Welling
mesh_terms: D000409:Alanine / Q000737:chemistry; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D001120:Arginine / Q000737:chemistry; D015153:Blotting, Western; D020126:Brefeldin A / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004594:Electrophysiology; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006031:Glycosylation; D006056:Golgi Apparatus / Q000378:metabolism; D047468:Immunoprecipitation; D007685:Kidney Tubules, Collecting / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D009865:Oocytes / Q000378:metabolism; D010766:Phosphorylation; D010768:Phosphoserine / Q000737:chemistry; D011188:Potassium / Q000378:metabolism; D024661:Potassium Channels, Inwardly Rectifying / Q000737:chemistry* / Q000502:physiology; D017434:Protein Structure, Tertiary; D018075:RNA, Complementary / Q000378:metabolism; D051381:Rats; D012694:Serine / Q000737:chemistry; D013997:Time Factors; D014982:Xenopus laevis
chemical_list: C510692:Kcnj1 protein, rat; D024661:Potassium Channels, Inwardly Rectifying; D018075:RNA, Complementary; D010768:Phosphoserine; D020126:Brefeldin A; D012694:Serine; D001120:Arginine; D000409:Alanine; D011188:Potassium