Molecular basis for the inactivation of Ca²⁺- and voltage-dependent BK channels in adrenal chromaffin cells and rat insulinoma tumor cells

Large-conductance Ca²⁺- and voltage-dependent potassium (BK) channels exhibit functional diversity not explained by known splice variants of the single Slo α-subunit. Here we describe an accessory subunit (β3) with homology to other β-subunits of BK channels that confers inactivation when it is coexpressed with Slo. Message encoding the β3 subunit is found in rat insulinoma tumor (RINm5f) cells and adrenal chromaffin cells, both of which express inactivating BK channels. Channels resulting from coexpression of Slo α and β3 subunits exhibit properties characteristic of native inactivating BK channels. Inactivation involves multiple cytosolic, trypsin-sensitive domains. The time constant of inactivation reaches a limiting value ~25-30 msec at Ca²⁺ of 10 μM and positive activation potentials. Unlike Shaker N- terminal inactivation, but like native inactivating BK channels, a cytosolic channel blocker does not compete with the native inactivation process. Finally, the β3 subunit confers a reduced sensitivity to charybdotoxin, as seen with native inactivating BK channels. Inactivation arises from the N terminal of the β3 subunit. Removal of the β3 N terminal (33 amino acids) abolishes inactivation, whereas the addition of the β3 N terminal onto the β1 subunit confers inactivation. The β3 subunit shares with the β1 subunit an ability to shift the range of voltages over which channels are activated at a given Ca²⁺. Thus, the β-subunit family of BK channels regulates a number of critical aspects of BK channel phenotype, including inactivation and apparent Ca²⁺ sensitivity.