Computational Insights Into Voltage Dependence Of Polyamine Block In Inwardly Rectifying K+ Channels

Inwardly rectifying potassium (Kir) channels play important roles in control of cellular excitability and K+ ion homeostasis. Under physiological conditions, Kir channels allow large K+ influx at potentials negative to the equilibrium potential of K+ but permit little outward current at potentials positive to the equilibrium potential of K+, due to voltage-dependent block of outward K+ flux by cytoplasmic polyamines. These polycationic molecules enter the Kir channel pore from the intracellular side. They block K+ ion movement through the channel at depolarized potentials, thereby ensuring, for instance, the long plateau phase of the cardiac action potential. Key questions concerning how deeply these charged molecules migrate into the pore and how the steep voltage dependence arises remain unclear. Recent MD simulations on GIRK2 crystal structures have provided unprecedented details concerning the conduction mechanism of the first Kir channel. Here, we use multi-microsecond-timescale MD simulations with applied field to provide detailed insights into voltage dependent block of putrescine, using the conductive state of the strong inwardly rectifying GIRK2 channel as starting point. Our µs long simulations suggest that steep rectification arises from movement of putrescine toward the selectivity filter site S4, thereby moving K+ ions through the selectivity filter. The simulations further elucidate details about a shallow binding site in the cytoplasmic domain and deep binding sites, in the pore.