Alkali metal cations modulate the geometry of different binding sites in HCN4 selectivity filter for permeation or block

Hyperpolarization-activated cyclic-nucleotide gated (HCN) channels are important for timing biological processes like heartbeat and neuronal firing. Their weak cation selectivity is determined by a filter domain with only two binding sites for K(+) and one for Na(+). The latter acts as a weak blocker, which is released in combination with a dynamic widening of the filter by K(+) ions, giving rise to a mixed K(+)/Na(+) current. Here, we apply molecular dynamics simulations to systematically investigate the interactions of five alkali metal cations with the filter of the open HCN4 pore. Simulations recapitulate experimental data like a low Li(+) permeability, considerable Rb(+) conductance, a block by Cs(+) as well as a punch through of Cs(+) ions at high negative voltages. Differential binding of the cation species in specific filter sites is associated with structural adaptations of filter residues. This gives rise to ion coordination by a cation-characteristic number of oxygen atoms from the filter backbone and solvent. This ion/protein interplay prevents Li(+), but not Na(+), from entry into and further passage through the filter. The site equivalent to S3 in K(+) channels emerges as a preferential binding and presumably blocking site for Cs(+). Collectively, the data suggest that the weak cation selectivity of HCN channels and their block by Cs(+) are determined by restrained cation-generated rearrangements of flexible filter residues.