Polyamine blockade and binding energetics in the MthK potassium channel

Polyamines such as spermidine and spermine are found in nearly all cells, at concentrations ranging up to 0.5 mM. These cations are endogenous regulators of cellular K(+) efflux, binding tightly in the pores of inwardly rectifying K(+) (K(ir)) channels in a voltage-dependent manner. Although the voltage dependence of K(ir) channel polyamine blockade is thought to arise at least partially from the energetically coupled movements of polyamine and K(+) ions through the pore, the nature of physical interactions between these molecules is unclear. Here we analyze the polyamine-blocking mechanism in the model K(+) channel MthK, using a combination of electrophysiology and computation. Spermidine (SPD(3+)) and spermine (SPM(4+)) each blocked current through MthK channels in a voltage-dependent manner, and blockade by these polyamines was described by a three-state kinetic scheme over a wide range of polyamine concentrations. In the context of the scheme, both SPD(3+) and SPM(4+) access a blocking site with similar effective gating valences (0.84 ± 0.03 e(0) for SPD(3+) and 0.99 ± 0.04 e(0) for SPM(4+)), whereas SPM(4+) binds in the blocked state with an ∼20-fold higher affinity than SPD(3+) (K(d) = 28.1 ± 3.1 µM for SPD(3+) and 1.28 ± 0.20 µM for SPM(4+)), consistent with a free energy difference of 1.8 kcal/mol. Molecular simulations of the MthK pore in complex with either SPD(3+) or SPM(4+) are consistent with the leading amine interacting with the hydroxyl groups of T59, at the selectivity filter threshold, with access to this site governed by outward movement of K(+) ions. These coupled movements can account for a large fraction of the voltage dependence of blockade. In contrast, differences in binding energetics between SPD(3+) and SPM(4+) may arise from distinct electrostatic interactions between the polyamines and carboxylate oxygens on the side chains of E92 and E96, located in the pore-lining helix.