Electrostatic interaction of internal Mg²⁺ with membrane PIP₂ seen with KCNQ K⁺ channels

Activity of KCNQ (Kv7) channels requires binding of phosphatidylinositol 4,5-bisphosphate (PIP₂) from the plasma membrane. We give evidence that Mg²⁺ and polyamines weaken the KCNQ0 channel-phospholipid interaction. Lowering internal Mg²⁺ augmented inward and outward KCNQ currents symmetrically, and raising Mg²⁺ reduced currents symmetrically. Polyvalent organic cations added to the pipette solution had similar effects. Their potency sequence followed the number of positive charges: putrescine (+2) < spermidine (+3) < spermine (+4) < neomycin (+6) < polylysine (≫+6). The inhibitory effects of Mg²⁺ were reversible with sequential whole-cell patching. Internal tetraethylammonium ion (TEA) gave classical voltage-dependent block of the pore with changes of the time course of K⁺ currents. The effect of polyvalent cations was simpler, symmetric, and without changes of current time course. Overexpression of phosphatidylinositol 4-phosphate 5-kinase Iγ to accelerate synthesis of PIP₂ attenuated the sensitivity to polyvalent cations. We suggest that Mg²⁺ and other polycations reduce the currents by electrostatic binding to the negative charges of PIP₂, competitively reducing the amount of free PIP₂ available for interaction with channels. The dose-response curves could be modeled by a competition model that reduces the pool of free PIP₂. This mechanism is likely to modulate many other PIP₂-dependent ion channels and cellular processes.