Accessory Beta Subunits Alter the Interaction Between the Membrane-Spanning and Cytosolic Domains in BK Channels

Abstract

Large conductance, Ca2+ and voltage-dependent K+ (BK) channels modulate physiology processes such as smooth muscle contraction and neurotransmission. BK channels are composed of the pore-forming α subunit (slo1) and four types of auxiliary β subunits (β1-4). While these β subunits show structural homology, they regulate gating properties of BK channels with distinct characteristics. For instance, the β1 subunit targets the membrane-spanning voltage sensor domain (VSD), while the β2 subunit targets the cytosolic domain (CTD) of slo1 to enhance Ca2+ sensitivity. Here we study how these β subunits perform a similar function by affecting two different structural domains of slo1. Using patch clamp techniques to study BK channels expressed in Xenopus oocytes, we found that both β1 and β2ND (the N-terminal residues 2-20 deleted to eliminate inactivation) alter channel activation by Mg2+ that is bound at the interface between VSD and CTD, suggesting that both β subunits alter the interaction between the two domains. Our data also show that a disulfide bond between residues 99C (in VSD) and 397C (in CTD) forms in the absence but not in the presence of β subunits, indicating that the alignment between the VSD and CTD is changed by both β subunits. Conversely, two mutations, N172R and N172RE399R, which have been found to alter the alignment between the VSD and CTD by introducing electrostatic forces between the two structural domains, change the effects of both β subunits on BK channel activation. Taken together, these results suggest that both β1 and β2 subunits alter the interactions between the VSD and the CTD by targeting one of the domains respectively to activate BK channels. This mechanism might be common for different β subunits modulating BK channel gating.