Insight into the Selectivity of Kir3.2 toward Phosphatidylinositides

Activation of G-protein-gated inwardly rectifying potassium channels (Kir3.x) requires the direct binding of phosphorylated phosphatidylinositides (PIPs). Previous studies have established that PIP isoforms activate Kir channels to varying degrees and the binding affinity between PIPs and Kir3.2 appears to be correlated with the level of activation. However, how individual residues contribute to the selectivity of Kir channels toward PIP isoforms is poorly understood. Here, we employ native mass spectrometry (MS) and fluorescent lipid binding assays to gain insight into the contribution of specific Kir3.2 residues binding to phospholipids. For the wild-type channel, we demonstrate the importance of membrane protein samples devoid of co-purified contaminants for protein-lipid binding studies and show that PIP(4,5)P-2 cooperatively binds Kir3.2 with a Hill coefficient of 2.7. We also find lipid binding profiles determined from native MS and solution binding assays are in direct agreement. Point mutations of Kir3.2 residues that interact with PIPs distinctly alter selective lipid binding. The K64Q mutation results in altered binding profiles with the highest binding affinity for PIP(4,5)P-2 with specific acyl chains. Mutation of R92 to Pro, a residue found in Kir6.2, results in promiscuous binding of PIP isoforms. Kir3.2 with the K194A mutation results in a distinct binding preference for PIP(3,4,5)P-3 over other PIP isoforms. Taken together, our results underscore the utmost importance of protein quality for protein-lipid binding studies and show that a single mutation in Kir3.2 can alter the selectivity toward PIPs.