A phosphorylation-dependent export structure in ROMK (Kir 1.1) channel overrides an endoplasmic reticulum localization signal

Dana Yoo, Liang Fang, Amanda Mason, Bo Young Kim, Paul A. Welling

Research output: Contribution to journalArticlepeer-review

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 glycosy lation 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.

Original languageEnglish (US)
Pages (from-to)35281-35289
Number of pages9
JournalJournal of Biological Chemistry
Volume280
Issue number42
DOIs
StatePublished - Oct 21 2005
Externally publishedYes

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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