Abstract
The β1 subunit of BK (large conductance Ca
2
+
and
voltage-activated K
+
) channels is essential for many key
physiological processes, such as controlling the contraction of smooth muscle
and the tuning of hair cells in the cochlea. Although it is known that the
β1 subunit greatly increases the open probability of BK channels, little
is known about its mechanism of action. We now explore this mechanism by using
channels in which the Ca
2
+
- and
Mg
2
+
-dependent activating mechanisms have been disrupted
by mutating three sites to remove the Ca
2
+
and
Mg
2
+
sensitivity. We find that the presence of the
β1 subunit partially restores Ca
2
+
sensitivity to
the triply mutated channels, but not the Mg
2
+
sensitivity. We also find that the β1 subunit has no effect on the
Mg
2
+
sensitivity of WT BK channels, in contrast to its
pronounced effect of increasing the apparent Ca
2
+
sensitivity. These observations suggest that the β1 subunit increases
open probability by working through the Ca
2
+
-dependent,
rather than Mg
2
+
-dependent, activating mechanisms, and
that the action of the β1 subunit is not directly on the
Ca
2
+
binding sites, but on the allosteric machinery
coupling the sites to the gate. The differential effects of the β1
subunit on the Ca
2
+
and Mg
2
+
activation of the channel suggest that these processes act separately.
Finally, we show that
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inhibits,
rather than activates, BK channels in the presence of the β1 subunit for
intermediate levels of
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. This
Mg
2
+
inhibition in the presence of the β1 subunit
provides an additional regulatory mechanism of BK channel activity.
