Selective Binding of a Toxin and Phosphatidylinositides to a Mammalian Potassium Channel

G-protein-gated inward rectifying potassium channels (GIRKs) require Gβγ subunits and phosphorylated phosphatidylinositides (PIPs) for gating. Although studies have provided insight into these interactions, the mechanism of how these events are modulated by Gβγ and the binding affinity between PIPs and GIRKs remains poorly understood. Here, native ion mobility mass spectrometry is employed to directly monitor small molecule binding events to mouse GIRK2. GIRK2 binds the toxin tertiapin Q and PIPs selectively and with significantly higher affinity than other phospholipids. A mutation in GIRK2 that causes a rotation in the cytoplasmic domain, similarly to Gβγ-binding to the wild-type channel, revealed differences in the selectivity towards PIPs More specifically, PIP isoforms known to weakly activate GIRKs have decreased binding affinity. Additionally, denaturing mass spectrometry and tryptic digest liquid chromatography mass spectrometry analysis was performed to confirm phosphorylation to GIRK2.