Molecular basis of fast inactivation in voltage and Ca ²⁺ -activated K ⁺ channels: A transmembrane β-subunit homolog

Voltage-dependent and calcium-sensitive K + (MaxiK) channels are key regulators of neuronal excitability, secretion, and vascular tone because of their ability to sense transmembrane voltage and intracellular Ca 2+ . In most tissues, their stimulation results in a noninactivating hyperpolarizing K + current that reduces excitability. In addition to noninactivating MaxiK currents, an inactivating MaxiK channel phenotype is found in cells like chromaffin cells and hippocampal neurons. The molecular determinants underlying inactivating MaxiK channels remain unknown. Herein, we report a transmembrane β subunit (β2) that yields inactivating MaxiK currents on coexpression with the pore-forming α subunit of MaxiK channels. Intracellular application of trypsin as well as deletion of 19 N-terminal amino acids of the β2 subunit abolished inactivation of the α subunit. Conversely, fusion of these N-terminal amino acids to the noninactivating smooth muscle β1 subunit leads to an inactivating phenotype of MaxiK channels. Furthermore, addition of a synthetic N-terminal peptide of the β2 subunit causes inactivation of the MaxiK channel α subunit by occluding its K + -conducting pore resembling the inactivation caused by the “ball” peptide in voltage-dependent K + channels. Thus, the inactivating phenotype of MaxiK channels in native tissues can result from the association with different β subunits.