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 exhib it functional diversity not explained by known splice variants of the singl e Slo alpha-subunit. Here we describe an accessory subunit (beta 3) with ho mology to other beta-subunits of BK channels that confers inactivation when it is coexpressed with Slo. Message encoding the beta 3 subunit is found i n rat insulinoma tumor (RINm5f) cells and adrenal chromaffin cells, both of which express inactivating BK channels. Channels resulting from coexpressi on of Slo alpha and beta 3 subunits exhibit properties characteristic of na tive inactivating BK channels. Inactivation involves multiple cytosolic, tr ypsin-sensitive domains. The time constant of inactivation reaches a limiti ng value similar to 25-30 msec at Ca2+ of 10 mu M and positive activation p otentials. Unlike Shaker N-terminal inactivation, but like native inactivat ing BK channels, a cytosolic channel blocker does not compete with the nati ve inactivation process. Finally, the beta 3 subunit confers a reduced sens itivity to charybdotoxin, as seen with native inactivating BK channels. Ina ctivation arises from the N terminal of the beta 3 subunit. Removal of the beta 3 N terminal (33 amino acids) abolishes inactivation, whereas the addi tion 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 aspec ts of BK channel phenotype, including inactivation and apparent Ca2+ sensit ivity.