β1 subunits facilitate gating of BK channels by acting through the Ca2+, but not the Mg2+, activating mechanisms

Xiang Qian, Karl L. Magleby

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31 Scopus citations

Abstract

The β1 subunit of BK (large conductance Ca2+ 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 Ca2+- and Mg2+-dependent activating mechanisms have been disrupted by mutating three sites to remove the Ca 2+ and Mg2+ sensitivity. We find that the presence of the β1 subunit partially restores Ca2+ sensitivity to the triply mutated channels, but not the Mg2+ sensitivity. We also find that the β1 subunit has no effect on the Mg2+ sensitivity of WT BK channels, in contrast to its pronounced effect of increasing the apparent Ca2+ sensitivity. These observations suggest that the β1 subunit increases open probability by working through the Ca 2+-dependent, rather than Mg2+-dependent, activating mechanisms, and that the action of the β1 subunit is not directly on the Ca2+ binding sites, but on the allosteric machinery coupling the sites to the gate. The differential effects of the β1 subunit on the Ca2+ and Mg2+ activation of the channel suggest that these processes act separately. Finally, we show that Mgi 2+ inhibits, rather than activates, BK channels in the presence of the β1 subunit for intermediate levels of Cai2+. This Mg2+ inhibition in the presence of the β1 subunit provides an additional regulatory mechanism of BK channel activity.

Original languageEnglish (US)
Pages (from-to)10061-10066
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume100
Issue number17
DOIs
StatePublished - Aug 19 2003

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