Cryo-EM structure of Slo1 with the auxiliary γ1 subunit suggests mechanism of depolarization-independent activation

Mammalian Ca²⁺-dependent Slo K⁺channels are expressed with β and γ auxiliary subunits that greatly influence voltage- and Ca²⁺-induced gating, thereby fundamentally altering the behavior of the channel. The four γ subunits reduce the need for voltage-dependent activation, allowing Slo to open in the absence of an action potential. The mechanism of this activation has, however, remained elusive. Here, we present the cryo-EM structure of Slo1 in complex with γ1/LRRC26, revealing how the transmembrane helix of γ1 binds and presumably stabilizes the active conformation of the voltage-sensor domain. This effect is further enhanced by a polybasic stretch on the intracellular side of the membrane which locally changes the charge gradient across the membrane. Sequence differences explain why the four γ subunits possess different activation efficiencies. Simultaneous binding of γ and the unrelated β subunits is structurally possible, as both binding sites do not overlap and the γ1 LRR domains are partially flexible. Thus, our data provide a possible explanation for Slo1 regulation by γ subunits, and furthermore suggest a novel mechanism of activation of voltage-gated ion channels by auxiliary subunits and add to the growing knowledge of their complex regulation.