Abstract
G-protein-gated inward rectifier potassium (GIRK) channels are regulated by G proteins and PIP2. Here, using cryo-EM single particle analysis we describe the equilibrium ensemble of structures of neuronal GIRK2 as a function of the C8-PIP2 concentration. We find that PIP2 shifts the equilibrium between two distinguishable structures of neuronal GIRK (GIRK2), extended and docked, towards the docked form. In the docked form the cytoplasmic domain, to which Gβγ binds, becomes accessible to the cytoplasmic membrane surface where Gβγ resides. Furthermore, PIP2 binding reshapes the Gβγ binding surface on the cytoplasmic domain, preparing it to receive Gβγ. We find that cardiac GIRK (GIRK1/4) can also exist in both extended and docked conformations. These findings lead us to conclude that PIP2 influences GIRK channels in a structurally similar manner to Kir2.2 channels. In Kir2.2 channels, the PIP2-induced conformational changes open the pore. In GIRK channels, they prepare the channel for activation by Gβγ.
Highlights
The inward rectifier K+ (Kir) channels were originally named for their rectifying current-voltage relationship (Hagiwara et al, 1976; Hagiwara and Takahashi, 1974; Hodgkin and Horowicz, 1959; Noble, 1965)
In the absence of PIP2 it is clear that GIRK2 can adopt a conformation in which the cytoplasmic domain (CTD) is disengaged from the TMD and the TMD-CTD linker has to be extended to account for the separation between the TMD and CTD
The main conclusion of this study is that GIRK channels – GIRK2 as well as GIRK1/4 – can adopt extended and docked conformations
Summary
The inward rectifier K+ (Kir) channels were originally named for their rectifying current-voltage relationship (Hagiwara et al, 1976; Hagiwara and Takahashi, 1974; Hodgkin and Horowicz, 1959; Noble, 1965) Today, this class of ion channels is defined by characteristic structural features encoded by the Kir family of related genes (Hibino et al, 2010). Kir channels underlie many physiological processes, including neuronal electrical activity, electrolyte homeostasis in the kidney, insulin secretion, and heart rate control (Hibino et al, 2010). Crystal structures of GIRK2 in the absence and presence of PIP2 (and in the presence of Gbg) did not show a change in the relationship between the CTD and TMD as was seen in Kir2.2 We study the structural effects of PIP2 on neuronal GIRK2 and cardiac GIRK1/4 using cryo-electron microscopy (cryo-EM) and correlate these effects with known properties of PIP2 activation
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