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 alpha-subunit. Here we describe an accessory subunit (beta 3) with homology to other beta-subunits of BK channels that confers inactivation when it is coexpressed with Slo. Message encoding the beta 3 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 alpha and beta 3 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 similar to 25-30 msec at Ca2+ of 10 mu M 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 beta 3 subunit confers a reduced sensitivity to charybdotoxin, as seen with native inactivating BK channels. Inactivation arises from the N terminal of the beta 3 subunit. Removal of the beta 3 N terminal (33 amino acids) abolishes inactivation, whereas the addition of the beta 3 N terminal onto the beta 1 subunit confers inactivation, The beta 3 subunit shares with the beta 1 subunit an ability to shift the range of voltages over which channels are activated at a given Ca2+. Thus, the beta-subunit family of BK channels regulates a number of critical aspects of BK channel phenotype, including inactivation and apparent Ca2+ sensitivity.