Modulation of potassium channels by transmembrane auxiliary subunits via voltage-sensing domains

Voltage-gated K+ (KV ) and Ca2+ -activated K+ (KCa ) channels are essential proteins for membrane repolarization in excitable cells. They also play important physiological roles in non-excitable cells. Their diverse physiological functions are in part the result of their auxiliary subunits. Auxiliary subunits can alter the expression level, voltage dependence, activation/deactivation kinetics, and inactivation properties of the bound channel. KV and KCa channels are activated by membrane depolarization through the voltage-sensing domain (VSD), so modulation of KV and KCa channels through the VSD is reasonable. Recent cryo-EM structures of the KV or KCa channel complex with auxiliary subunits are shedding light on how these subunits bind to and modulate the VSD. In this review, we will discuss four examples of auxiliary subunits that bind directly to the VSD of KV or KCa channels: KCNQ1-KCNE3, Kv4-DPP6, Slo1-β4, and Slo1-γ1. Interestingly, their binding sites are all different. We also present some examples of how functionally critical binding sites can be determined by introducing mutations. These structure-guided approaches would be effective in understanding how VSD-bound auxiliary subunits modulate ion channels.

Analyses of the Kroon Virus Major Capsid Gene and Its Transcript Highlight a Distinct Pattern of Gene Evolution and Splicing among Mimiviruses

The inclusion of Mimiviridae members in the putative monophyletic nucleocytoplasmic large DNA virus (NCLDV) group is based on genomic and phylogenomic patterns. This shows that, along with other viral families, they share a set of genes known as core or "hallmark genes," including the gene for the major capsid protein (MCP). Although previous studies have suggested that the maturation of mimivirus MCP transcripts is dependent on splicing, there is little information about the processing of this transcript in other mimivirus isolates. Here we report the characterization of a new mimivirus isolate, called Kroon virus (KV) mimivirus. Analysis of the structure, synteny, and phylogenetic relationships of the MCP genes in many mimivirus isolates revealed a remarkable variation at position and types of intronic and exonic regions, even for mimiviruses belonging to the same lineage. In addition, sequencing of KV and Acanthamoeba polyphaga mimivirus (APMV) MCP transcripts has shown that inside the family, even related giant viruses may present different ways to process the MCP mRNA. These results contribute to the understanding of the genetic organization and evolution of the MCP gene in mimiviruses.IMPORTANCE Mimivirus isolates have been obtained by prospecting studies since 2003. Based on genomic and phylogenomic studies of conserved genes, these viruses have been clustered together with members of six other viral families. Although the major capsid protein (MCP) gene is an important member of the so-called "hallmark genes," there is little information about the processing and structure of this gene in many mimivirus isolates. In this work, we have analyzed the structure, synteny, and phylogenetic relationships of the MCP genes in many mimivirus isolates; these genes showed remarkable variation at position and types of intronic and exonic regions, even for mimiviruses belonging to the same lineage. These results contribute to the understanding of the genetic organization and evolution of the MCP gene in mimiviruses.

The roles of KCa, KATP, and KV channels in regulating cutaneous vasodilation and sweating during exercise in the heat

We recently showed the varying roles of Ca²⁺-activated (K_Ca), ATP-sensitive (K_ATP), and voltage-gated (K_V) K⁺ channels in regulating cholinergic cutaneous vasodilation and sweating in normothermic conditions. However, it is unclear whether the respective contributions of these K⁺ channels remain intact during dynamic exercise in the heat. Eleven young (23 ± 4 yr) men completed a 30-min exercise bout at a fixed rate of metabolic heat production (400 W) followed by a 40-min recovery period in the heat (35°C, 20% relative humidity). Cutaneous vascular conductance (CVC) and local sweat rate were assessed at four forearm skin sites perfused via intradermal microdialysis with: 1) lactated Ringer solution (control); 2) 50 mM tetraethylammonium (nonspecific K_Ca channel blocker); 3) 5 mM glybenclamide (selective K_ATP channel blocker); or 4) 10 mM 4-aminopyridine (nonspecific K_V channel blocker). Responses were compared at baseline and at 10-min intervals during and following exercise. K_Ca channel inhibition resulted in greater CVC versus control at end exercise (P = 0.04) and 10 and 20 min into recovery (both P < 0.01). K_ATP channel blockade attenuated CVC compared with control during baseline (P = 0.04), exercise (all P ≤ 0.04), and 10 min into recovery (P = 0.02). No differences in CVC were observed with K_V channel inhibition during baseline (P = 0.15), exercise (all P ≥ 0.06), or recovery (all P ≥ 0.14). With the exception of K_V channel inhibition augmenting sweating during baseline (P = 0.04), responses were similar to control with all K⁺ channel blockers during each time period (all P ≥ 0.07). We demonstrated that K_Ca and K_ATP channels contribute to the regulation of cutaneous vasodilation during rest and/or exercise and recovery in the heat.

Vasorelaxation of goat mesenteric artery is mediated by endothelial Na(+)-K(+)-ATPase

OBJECTIVE

To examine the role of Na(+)-K(+)-ATPase and K(+) channels in mediating vasorelaxation in the superior mesenteric artery of Capra hircus.

MATERIALS AND METHODS

Goat superior mesenteric artery (GSMA) was cut into 1.5-2 mm circular rings and mounted in a thermostatically controlled (37°C ± 0.5°C) organ bath containing 20 ml of modified Krebs-Henseleit saline (MKHS) (pH 7.4), with continuous aeration under 1.5 g tension for 90 min. Endothelium-intact (ED+) or endothelium-denuded (ED-) GSMA ring was contracted with phenylephrine (PE) or 5-hydroxytryptamine (5-HT) (1 μM-0.1 mM) in the absence or presence of ouabain (0.1 μM). KCl (1 μM-10 mM) was added cumulatively to K(+)-free MKHS-pre-contracted (ED+/-) rings in the absence or presence of ouabain (0.1 μM) or barium (1 μM) or 4-aminopyridine (1 μM).

RESULTS

Ouabain did not alter the basal tone of the arterial ring. The contractile response induced by PE (Emax: 50.46 ± 2.68, pD2: 5.53 ± 0.04) and 5-HT (Emax: 30.86 ± 1.33, pD2: 6.17 ± 0.03) in ED+ ring was significantly (P < 0.001) augmented in ED- rings (PE: Emax: 93.30 ± 2.11, pD2: 6.41 ± 0.04; 5-HT: Emax: 95.07 ± 0.99, pD2: 6.27 ± 0.03). The contractile response induced by PE and 5-HT in ED+ or ED- rings in the presence of ouabain was almost identical with that of ED- rings. Vasorelaxation of KCl (Emax: 2.90 ± 1.14, pD2: 3.9 ± 0.03) was significantly attenuated in the presence of ouabain (Emax: 73.8 ± 5.16, pD2: 4.3 ± 0.04), Ba(2+) (Emax: 16.34 ± 4.7, pD2: 3.22 ± 0.02), 4-AP (Emax: 18.16 ± 2.4, pD2: 3.68 ± 0.03), ouabain and Ba(2+) (Emax: 70.09 ± 3.66, pD2: 4.41 ± 0.04), and ouabain and 4-AP (Emax: 66.98 ± 4.61, pD2: 4.13 ± 0.06).

CONCLUSION

The vasorelaxation in GSMA is mediated by the endothelium-derived hyperpolarizing factor (EDHFs) such as ouabain-sensitive Na(+)-K(+)-ATPase, KIR and Kv channels.

Regulation of Voltage-Activated K(+) Channel Gating by Transmembrane β Subunits

Voltage-activated K⁺ (K_V) channels are important for shaping action potentials and maintaining resting membrane potential in excitable cells. K_V channels contain a central pore-gate domain (PGD) surrounded by four voltage-sensing domains (VSDs). The VSDs will change conformation in response to alterations of the membrane potential thereby inducing the opening of the PGD. Many K_V channels are heteromeric protein complexes containing auxiliary β subunits. These β subunits modulate channel expression and activity to increase functional diversity and render tissue specific phenotypes. This review focuses on the K_V β subunits that contain transmembrane (TM) segments including the KCNE family and the β subunits of large conductance, Ca²⁺- and voltage-activated K⁺ (BK) channels. These TM β subunits affect the voltage-dependent activation of K_V α subunits. Experimental and computational studies have described the structural location of these β subunits in the channel complexes and the biophysical effects on VSD activation, PGD opening, and VSD–PGD coupling. These results reveal some common characteristics and mechanistic insights into K_V channel modulation by TM β subunits.