Molecular design of the MaxiK channel. A: proposed topology ofα- and β-subunits. The channel is formed by 4α-subunits and probably 4 β-subunits. B: alignment of S4 segments of the 3 members of Slo family. Note that Slo2 lacks positive charges in this segment. C: alignment of the region located at beginning of S10 segment. The Ca2+ bowl present in Slo1 is absent in Slo2 and Slo3.
Modulation of voltage and Ca2+ dependences given by β-subunits. A: current traces elicited by a voltage pulse of 150 mV at 4 μM intracellular Ca2+ for channels formed byα-,α + β1-, andα + β4-subunits. B: open probability-voltage (Po-V) relationships at 4 μM (filled symbols) and 7 nM (open symbols) intracellular Ca2+ concentration for channels formed byα- (squares),α + β1- (circles), andα + β4- (triangles) subunits.
Proposed physiological roles of MaxiK channel β-subunits, depicted as cartoons (A) and as examples on the impact of β-subunits on channel electrophysiology (B). In vascular smooth muscle (VSM) cells, β1-subunit confers the required Ca2+ sensitivity for effective coupling between Ca2+ sparks and spontaneous outward currents. In chromaffin cells, slowed MaxiK deactivation kinetics allow β2-subunit-expressing cells to fire repeatedly. In auditory outer hair cells, a combination of MaxiK splice variants and β1-subunit expression gives rise to a variety of MaxiK channel kinetics, allowing each cell to electrically tune to a characteristic frequency. See text for further details. B, left: reproduced with permission from Ref. 3. B, middle: reproduced with permission from Ref. 14. B, right, reproduced with permission from Ref. 5