Stoichiometry of the cardiac I Ks complex

Nothing is easy for ion channels that assemble with membrane-embedded regulatory subunits. Something as trivial yet as fundamental as counting the number of subunits in an ion channel complex can be challenging and marred with controversy. The most infamous stoichiometric debate in the ion channel field has been over the potassium (K+) channel complex that generates the repolarizing cardiac IKslow (IKs) current (1⇓⇓⇓–5). This union between a tetrameric voltage-gated K+ channel (KvLQT1, KCNQ1, Kv7.1) and type I transmembrane regulatory subunit (minK, KCNE1) is unquestioned; however, the last stoichiometric salvo in the debate proposed a haphazard coassembly of cardiac IKs complexes, containing one to four regulatory subunits (4). Determining the exact number of regulatory subunits in the cardiac IKs complex is vital because mutations in either the ion conducting or regulatory subunit that reduce potassium flow give rise to Long QT and Jervell and Lange-Nielsen syndromes—two diseases where individuals are prone to adrenaline-induced, life-threatening cardiac arrhythmias (6, 7). Attempts to find small molecule openers that enhance cardiac repolarization by targeting the K+ channel (8, 9) have been ironically thwarted by the presence of the regulatory subunit whose stoichiometry in the complex is continually in question. In PNAS, Plant et al. (10) warm up their laser beams and take aim at this cardiac potassium channel complex using fluorescent subunit photobleaching. This single molecule spectroscopy approach relies on the mathematically predictable photobleaching of a fluorophore (11), usually monomeric GFP (or a color variant), which is appended to each subunit of interest in an ion channel complex. Thus, the technique can be used for any membrane-embedded complex as long as enough fluorescent …