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Abstract: Objective: The voltage-gated K+ channel KCNQ1 associates with the small KCNE1 β subunit to underlie the IKs repolarizing current in the heart. Based on sequence homology, the KCNE family is recognized to comprise five members. Controversial data have indicated their participation in several K+ channel protein complexes, including KCNQ1. The expression level and the putative functions of the different KCNE subunits in the human heart still require further investigation. Methods: We have carried out a comparative study of all KCNE subunits with KCNQ1 using the patch-clamp technique in mammalian cells. Real-time RT-PCR absolute quantification was performed on human atrial and ventricular tissue. Results: While KCNQ1/KCNE1 heteromultimer reached high current density with slow gating kinetics and pronounced voltage dependence, KCNQ1/KCNE2 and KCNQ1/KCNE3 complexes produced instantaneous voltage-independent currents with low and high current density, respectively. Co-expression of KCNE4 or KCNE5 with KCNQ1 induced small currents in the physiological range of voltages, with kinetics similar to those of the KCNQ1/KCNE1 complex. However, co-expression of these inhibitory subunits with a disease-associated mutation (S140G-KCNQ1) led to currents that were almost undistinguishable from the KCNQ1/KCNE1 canonical complex. Absolute cDNA quantification revealed a relatively homogeneous distribution of each transcript, except for KCNE4, inside left atria and endo- and epicardia of left ventricular wall with the following abundance: KCNQ1≫KCNE4≥KCNE1>KCNE3>KCNE2>KCNE5. KCNE4 expression was twice as high in atrium compared to ventricle. Conclusions: Our data show that KCNQ1 forms a channel complex with 5 KCNE subunits in a specific manner but only interactions with KCNE1, KCNE2, and KCNE3 may have physiological relevance in the human heart. [Copyright &y& Elsevier]

The tandem pore domain halothane-inhibited K+ channel 1 (THIK1) produces backgroundK+ currents. Despite 62% amino acid identity with THIK1, THIK2 is not active upon heterologous expression. Here, we show that this apparent lack of activity is due to a unique combination of retention in the endoplasmic reticulum and low intrinsic channel activity at the plasma membrane. A THIK2 mutant containing a proline residue (THIK2-A155P) in its second inner helix (M2) produces K+-selective currents with properties similar to THIK1, including inhibition by halothane and insensitivity to extracellular pH variations. Another mutation in the M2 helix (I158D) further increases channel activity and affects current kinetics. We also show that the cytoplasmic amino-terminal region of THIK2 (Nt- THIK2) contains an arginine-rich motif (RRSRRR) that acts as a retention/retrieval signal. Mutation of this motif in THIK2 induces a relocation of the channel to the plasma membrane, resulting in measurable currents, even in the absence of mutations in the M2 helix. Cell surface delivery of a Nt-THIK2- CD161 chimera is increased by mutating the arginines of the retention motif but also by converting the serine embedded in this motif to aspartate, suggesting a phosphorylation-dependent regulation of THIK2 trafficking. [ABSTRACT FROM AUTHOR]