Ligand-Dependent Activation of Slo Family Channels Is Defined by Interchangeable Cytosolic Domains

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Abstract
Large-conductance Ca2+- and voltage-regulated K+ channels (Slo1 BK-type) are controlled by two physiological stimuli, membrane voltage and cytosolic Ca2+. Regulation by voltage is similar to that in voltage-dependent K+ channels, arising from positively charged amino acids primarily within the S4 transmembrane helices. The basis for regulation by Ca2+ remains controversial. One viewpoint suggests that the extensive cytosolic C terminus contains the Ca2+ regulatory machinery, whereas another suggests that the pore-forming module contains the Ca2+-sensing elements. To address this issue, we take advantage of another Slo family member, the pH-regulated homolog Slo3. We reason that if the ligand-sensing apparatus is uniquely associated with a particular domain (either the pore or the cytosolic domain), exchange of those domains between Slo1 and Slo3 should result in exchange of ligand dependence in association with the key domain. The results show that the Slo3 cytosolic module confers pH-dependent regulation on the Slo1 pore module, whereas the Slo1 cytosolic module confers Ca2+-dependent regulation on the Slo3 pore module. Thus, ligand-specific regulation is defined by interchangeable cytosolic regulatory modules.