Ball-and-chain inactivation in a calcium-gated potassium channel

Inactivation is the process by which ion channels terminate ion flux through their pores while the opening stimulus is still present1. In neurons, inactivation of both sodium and potassium channels is crucial for the generation of action potentials and regulation of firing frequency1,2. A cytoplasmic domain of either the channel or an accessory subunit is thought to plug the open pore to inactivate the channel via a 'ball-and-chain' mechanism3-7. Here we use cryo-electron microscopy to identify the molecular gating mechanism in calcium-activated potassium channels by obtaining structures of the MthK channel from Methanobacterium thermoautotrophicum-a purely calcium-gated and inactivating channel-in a lipid environment. In the absence of Ca2+, we obtained a single structure in a closed state, which was shown by atomistic simulations to be highly flexible in lipid bilayers at ambient temperature, with large rocking motions of the gating ring and bending of pore-lining helices. In Ca2+-bound conditions, we obtained several structures, including multiple open-inactivated conformations, further indication of a highly dynamic protein. These different channel conformations are distinguished by rocking of the gating rings with respect to the transmembrane region, indicating symmetry breakage across the channel. Furthermore, in all conformations displaying open channel pores, the N terminus of one subunit of the channel tetramer sticks into the pore and plugs it, with free energy simulations showing that this is a strong interaction. Deletion of this N terminus leads to functionally non-inactivating channels and structures of open states without a pore plug, indicating that this previously unresolved N-terminal peptide is responsible for a ball-and-chain inactivation mechanism.

Dynamin- and Rab5-dependent endocytosis of a Ca2+ -activated K+ channel, KCa2.3

Regulation of the number of ion channels at the plasma membrane is a critical component of the physiological response. We recently demonstrated that the Ca²⁺-activated K⁺ channel, KCa2.3 is rapidly endocytosed and enters a Rab35- and EPI64C-dependent recycling compartment. Herein, we addressed the early endocytic steps of KCa2.3 using a combination of fluorescence and biotinylation techniques. We demonstrate that KCa2.3 is localized to caveolin-rich domains of the plasma membrane using fluorescence co-localization, transmission electron microscopy and co-immunoprecipitation (co-IP). Further, in cells lacking caveolin-1, we observed an accumulation of KCa2.3 at the plasma membrane as well as a decreased rate of endocytosis, as assessed by biotinylation. We also demonstrate that KCa2.3 and dynamin II are co-localized following endocytosis as well as demonstrating they are associated by co-IP. Further, expression of K44A dynamin II resulted in a 2-fold increase in plasma membrane KCa2.3 as well as a 3-fold inhibition of endocytosis. Finally, we evaluated the role of Rab5 in the endocytosis of KCa2.3. We demonstrate that expression of a dominant active Rab5 (Q79L) results in the accumulation of newly endocytosed KCa2.3 on to the membrane of the Rab5-induced vacuoles. We confirmed this co-localization by co-IP; demonstrating that KCa2.3 and Rab5 are associated. As expected, if Rab5 is required for the endocytosis of KCa2.3, expression of a dominant negative Rab5 (S34N) resulted in an approximate 2-fold accumulation of KCa2.3 at the plasma membrane. This was confirmed by siRNA-mediated knockdown of Rab5. Expression of the dominant negative Rab5 also resulted in a decreased rate of KCa2.3 endocytosis. These results demonstrate that KCa2.3 is localized to a caveolin-rich domain within the plasma membrane and is endocytosed in a dynamin- and Rab5-dependent manner prior to entering the Rab35/EPI64C recycling compartment and returning to the plasma membrane.

Single plasma membrane K+ channel detection by using dual-color quantum dot labeling

K+ channels are widely expressed in eukaryotic and prokaryotic cells, where one of their key functions is to set the membrane potential. Many K+ channels are tetramers that share common architectural properties. The crystal structure of bacterial and mammalian K+ channels has been resolved and provides the basis for modeling their three-dimensional structure in different functional states. This wealth of information on K+ channel structure contrasts with the difficulties to visualize single K+ channel proteins in their physiological environment. We describe a method to identify single Ca2+-activated K+ channel molecules in the plasma membrane of migrating cells. Our method is based on dual-color labeling with quantum dots. We show that >90% of the observed quantum dots correspond to single K+ channel proteins. We anticipate that our method can be adopted to label any other ion channel in the plasma membrane on the single molecule level.

Calcium-activated potassium channels and the regulation of vascular tone

Different calcium signals in the endothelium and smooth muscle target different types of Ca2+-sensitive K+ channels to modulate vascular function. These differential calcium signals and targets represent multilayered opportunities for prevention and/or treatment of vascular dysfunctions.

Distribution of Ca2+-activated K+ channel (SK2 and SK3) immunoreactivity in intestinal smooth muscles of the guinea-pig

1. The tissue distribution of small conductance Ca(2+)-activated K(+) channels (SK2 and SK3) was examined in three preparations of the guinea-pig intestine: the taneia caeci and the circular muscle layer of the stomach and proximal colon. 2. The SK3 immunoreactive (SK3-IR) cells were bi- or multipolar in appearance with numerous short processes, which formed an interconnecting network at the myenteric and submucous borders of the stomach and proximal colon. The SK3-IR cells were also present within the circular muscle layer of these preparations and throughout the taenia caeci. 3. Although SK3-IR cells had a similar distribution as cells immunoreactive for c-Kit (c-Kit-IR), the marker for interstitial cells of Cajal (ICC), only 5-10% of c-Kit-IR ICC were also SK3-IR. 4. The SK3-IR cells were clearly ICC when examined with the electron microscope. Close associations of SK3-IR ICC (ICC-SK3) and nerves were often observed, as were gap junctions with SK3-negative ICC and smooth muscle cells. 5. Punctate SK2 and SK3 channel immunoreactivity was present on the plasmalemmal surface of all smooth muscle cells examined. 6. We conclude that ICC-SK3 are a subpopulation of ICC that are directly innervated by enteric inhibitory motor nerves.