Structure, Gating and Basic Functions of the Ca2+-activated K Channel of Intermediate Conductance

Abstract

Background: The KCa3.1 channel is the intermediate-conductance member of the Ca2+- activated K channel superfamily. It is widely expressed in excitable and non-excitable cells, where it plays a major role in a number of cell functions. This paper aims at illustrating the main structural, biophysical and modulatory properties of the KCa3.1 channel, and providing an account of experimental data on its role in volume regulation and Ca2+ signals.

Methods: Research and online content related to the structure, structure/function relationship, and physiological role of the KCa3.1 channel are reviewed.

Results: Expressed in excitable and non-excitable cells, the KCa3.1 channel is voltage independent, its opening being exclusively gated by the binding of intracellular Ca2+ to calmodulin, a Ca2+- binding protein constitutively associated with the C-terminus of each KCa3.1 channel α subunit. The KCa3.1 channel activates upon high affinity Ca2+ binding, and in highly coordinated fashion giving steep Hill functions and relatively low EC50 values (100-350 nM). This high Ca2+ sensitivity is physiologically modulated by closely associated kinases and phosphatases. The KCa3.1 channel is normally activated by global Ca2+ signals as resulting from Ca2+ released from intracellular stores, or by the refilling influx through store operated Ca2+ channels, but cases of strict functional coupling with Ca2+-selective channels are also found. KCa3.1 channels are highly expressed in many types of cells, where they play major roles in cell migration and death. The control of these complex cellular processes is achieved by KCa3.1 channel regulation of the driving force for Ca2+ entry from the extracellular medium, and by mediating the K+ efflux required for cell volume control.

Conclusion: Much work remains to be done to fully understand the structure/function relationship of the KCa3.1 channels. Hopefully, this effort will provide the basis for a beneficial modulation of channel activity under pathological conditions.

Keywords: KCa3.1; NDPK-B; PKA; calcium influx; calmodulin; gating; volume regulation..

Fig. (1)

Biophysical properties of KCa3.1 channel in human glioblastoma GL-15 cells. A) representative single-channel recordings at varying voltages in symmetrical 150 mM K⁺ and 0.3 μM [Ca²⁺]_i. B) current-voltage relationship constructed through a double Gaussian fit of the current amplitude histograms made from the unitary currents of the type shown in panel A, under symmetrical 150 mM K⁺ (□) and after substitution of internal K⁺ with Na⁺ (○). Dashed line, representing linear fit of the 3 control data points at the most negative voltages, gives a slope conductance (γ) of 25 pS for this channel. Inset: open probability (Po) vs. voltage for the patch shown in A. C) inside-out single-channel recordings at −100 mV in symmetrical K⁺ conditions at the indicated [Ca²⁺]_i. D) KCa3.1 channel Po vs. [Ca²⁺]_i. The solid line represents the best fit of the experimental data with a Hill function (best-fit parameters are indicated) From [7].

Fig. (2)

Structure and membrane topology of the KCa3.1 channel. KCa3.1 is a tetrameric protein, with each subunit organized in six transmembrane segments plus a pore region between segments 5 and 6. The channel Ca²⁺ sensitivity is conferred by CAM, with the CAM C-lobe constitutively bound to the 312-329 segment of the channel C-terminal region (CAMBD1). The Ca²⁺-dependent binding of the CAM to the channel with the CAM N-lobe involves the segments 360 to 373 (CAMBD2B) and 344 to 353 (CAMBD2A). The proteins and channel residues relevant to the modulation of the channel by PKA, AMPK, and PI3P are shown.

Fig. (3)

Schematic illustrating the suggested Ca²⁺-induced dimerization of the intracellular C-terminal segments leading to KCa3.1 channel opening, as deduced from studies on the SKCa channel. The crystal structure of the SKCa channel obtained in the presence of Ca²⁺ ions indicates that the Ca²⁺/CAM/CAMBD complex forms a dimer structure in which two antiparallel C-terminal segments of two adjacent channel subunits are joined together by two CAMs.

Fig. (4)

A), Schematic of the role of KCa3.1 channels activity in controlling intracellular Ca²⁺ signals promoted by PLC-coupled receptors. Description in the text. B), Biphasic Ca²⁺ response upon prolonged application of histamine on a GL-15 glioblastoma cell, and effects of TRAM-34 on the sustained component. From [68].