Homomeric and Heteromeric Assembly of KCNQ (Kv7) K+ Channels Assayed by Total Internal Reflection Fluorescence/Fluorescence Resonance Energy Transfer and Patch Clamp Analysis

Abstract

M-type K؉ channels, consisting of KCNQ1-5 (Kv7.1-7.5) subunits, form a variety of homomeric and heteromeric channels. Whereas all the subunits can assemble into homomeric channels, the ability of the subunits to assemble into heteromultimers is highly variable. KCNQ3 is widely thought to co-assemble with several other KCNQ subtypes, whereas KCNQ1 and KCNQ2 do not. However, the existence of other subunit assemblies is not well studied. To systematically explore the heteromeric assembly of KCNQ channels in individual living cells, we performed fluorescence resonance energy transfer (FRET) between cyan fluorescent proteinand yellow fluorescent protein-tagged KCNQ subunits expressed in Chinese hamster ovary cells under total internal reflection fluorescence microscopy in which excitation light only penetrates several hundred nanometers into the cell, thus isolating membrane events. We found significant FRET between homomeric subunits as expected from their functional expression in heterologous expression systems. Also as expected from previous work, robust FRET was observed between KCNQ2 and KCNQ3. KCNQ3 and KCNQ4 also showed substantial FRET as did KCNQ4 and KCNQ5. To determine functional assembly of KCNQ4/KCNQ5 heteromers, we performed two types of experiments. In the first, we constructed a mutant tetraethylammonium ion-sensitive KCNQ4 subunit and tested its assembly with KCNQ5 by patch clamp analysis of the tetraethylammonium ion sensitivity of the resulting current; however, those data were not conclusive. In the second, we co-expressed a KCNQ4 (G285S) pore mutant with KCNQ5 and found the former to act as a dominant negative, suggesting co-assembly of the two types of subunits. These data confirm that among the allowed assembly conformations are KCNQ3/4 and KCNQ4/5 heteromers.The KCNQ (Kv7) family of voltage-gated channels underlie a number of important K ϩ currents throughout the body, including the M current of neurons produced by the homomeric and heteromeric assembly of KCNQ2, KCNQ3, and KCNQ5 (1-7). KCNQ4 makes homomeric K ϩ channels important for K ϩ transport primarily in the inner ear (8, 9), whereas KCNQ1 assembles with KCNE ␤-subunits to form several important currents in heart, ear, and epithelia (10, 11). The understanding of the different subunit compositions of KCNQ channels is important because such compositions affect the properties of the channels. Thus, expression of KCNQ2 and KCNQ3 individually yields only small whole-cell currents, whereas their coexpression yields heteromeric currents 10-fold larger (5, 12-16). Homomeric KCNQ2, -4, and -5 channels are sensitive to Ca 2ϩ /calmodulin, whereas KCNQ1 and KCNQ3 are not (17). Finally KCNQ1 and KCNQ2 subunits in particular have been shown to be modulated by phosphorylation by protein kinases A and C, respectively, via complexes assembled by the scaffolding proteins yotiao and AKAP79/150 (18 -21). Therefore, it is expected that the inclusion of these different subunits in the channel tetramer should confer specific properties characteristic of each...

Vestibular Role of KCNQ4 and KCNQ5 K+ Channels Revealed by Mouse Models

“…1, C and L, and supplemental Fig. S1, C and L) suggested that they may form heteromeric channels in vivo, as has been reported for co-transfected Chinese hamster ovary cells (24). Using dominant negative mutants, including the one expressed in Kcnq5 dn/dn mice, we confirmed these findings in the Xenopus oocyte system that allows a more quantitative control of co-expression levels (supplemental Fig.…”

Effects of KCNQ channel modulators on the M-type potassium current in primate retinal pigment epithelium

“…4C). Among homomeric KCNQ channels, KCNQ1 and KCNQ2 exhibit high sensitivity to block by extracellular TEA, whereas KCNQ3, KCNQ4, and KCNQ5 are relatively TEA-insensitive (2,22). The M-type current in the RPE was moderately sensitive to block by chromanol 293B, a KCNQ1 channel blocker (22).…”

“…4). KCNQ1 channels are sensitive to block by extracellular TEA in the millimolar range (6), whereas KCNQ5 channels are relatively TEA-insensitive, with IC 50 of 43 mM (2). It has been stated that the KCNQ4 channel has an IC 50 for block by TEA of 3 mM (6), but a more recent study reported an IC 50 of ϳ50 mM (2); the latter value is in agreement with our results testing the effect of TEA on hKCNQ4_v1 and hKCNQ4_v2 channels expressed in Chinese hamster ovary cells (unpublished data).…”

Ion channel regulation by β-secretase BACE1 – enzymatic and non-enzymatic effects beyond Alzheimer's disease

“…The pore-forming a-subunits form homo-or hetero-tetramers. 74,75 KCNQ1 (also: K V LQT, long QT) is expressed in heart, gastrointestinal tract, and inner ear. 76 KCNQ2 and KCNQ3 are widely found in the central and peripheral nervous system.…”