The single transmembrane segment determines the modulatory function of the BK channel auxiliary γ subunit

The large-conductance, calcium-activated potassium (BK) channels consist of the pore-forming, voltage- and Ca2+-sensing alpha subunits (BK alpha) and the tissue-specific auxiliary beta and gamma subunits. The BK channel gamma 1 subunit is a leucine-rich repeat (LRR)-containing membrane protein that potently facilitates BK channel activation in many tissues and cell types through a vast shift in the voltage dependence of channel activation by similar to 140 mV in the hyperpolarizing direction. In this study, we found that the single transmembrane (TM) segment together with its flanking charged residues is sufficient to fully modulate BK channels upon its transplantation into the structurally unrelated beta 1 subunit. We identified Phe273 and its neighboring residues in the middle of the TM segment and a minimum of three intracellular juxtamembrane Arg residues as important for the gamma 1 subunit's modulatory function and observed functional coupling between residues of these two locations. We concluded that the TM segment is a key molecular determinant for channel association and modulation and that the intracellular positively charged cluster is involved mainly in channel association, likely through its TM-anchoring effect. Our findings provide insights into the structure-function relationship of the gamma 1 subunit in understanding its potent modulatory effects on BK channels.