Ion channel-transporter interactions

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 DESCRIPTION (provided by applicant): The KCNQ1 voltage-gated potassium (Kv) channel pore-forming (a) subunit is ubiquitously expressed and linked to life-threatening human disorders including Long QT syndrome, atrial fibrillation and diabetes. KCNQ1 exhibits a high degree of functional flexibility enabled by co-assembly with KCNE family β subunits, facilitating roles both in excitable cell repolarization, and as a constitutively active K+ channel in polarized epithelial cells. Na+-coupled solute transport is crucial for uptake of ions and solutes including sugars and myo-inositol, an important osmolyte and precursor for cell signaling molecules. We recently discovered that KCNQ1 forms complexes with several different Na+-coupled solute transporters, and that at least two of these novel complexes are required for normal epithelial cell activity - in the thyroid and choroid plexus. Using in vitro functional studies, we found that KCNQ1 and the Na+-dependent myo-inositol transporters SMIT1 and SMIT2 reciprocally regulate each other's function. We also recently identified several other channel-transporter interactions, including KCNQ4-SMIT1, KCNQ1-SGLT1 (Na+-coupled glucose transporter), and KCNQ1-NIS (Na+/I- symporter). Here, we will elucidate molecular mechanisms of function, interaction, and physiological relevance of this novel and potentially widespread class of macromolecular signaling complexes. Three main questions are addressed. First, which channel domains and functions regulate transporter activity? Using mutagenesis, pharmacological agents and functional analyses, we will test the hypothesis that the KCNQ1 pore and voltage sensor modules can independently influence activity of co-assembled solute transporters. We will also use protein biochemistry in conjunction with channel chimeras and mutagenesis to elucidate channel domains crucial for physical interaction with transporters, in vitro. Second, why are channel-transporter complexes required? Using electrophysiological and solute uptake assays in vitro we will test the hypothesis that KCNQ1 acts as a biosensor in complexes with SMIT1 and SMIT2, facilitating responses to changes in osmolarity, membrane lipid composition, pH and Ca2+. Third, where do channel-transporter complexes occur in vivo? Aided by several knockout mouse lines, we will locate channel-transporter complexes, and utilize positron emission tomography to specifically test for the requirement of KCNQ1-KCNE regulation of SGLT family transporters, in vivo. Na+-coupled solute transporters, and Kv channel a subunits including KCNQ1, exhibit broad distribution, wide tissue expression overlap, and high biomedical significance. Together with our recent findings, this suggests that potassium channel-transporter complexes have the potential to be highly influential in mammalian physiology and in the pathogenesis of a number of prevalent human disorders.