title: Pore dimensions and the role of occupancy in unitary conductance of Shaker K channels.
abstract: K channels mediate the selective passage of K(+) across the plasma membrane by means of intimate interactions with ions at the pore selectivity filter located near the external face. Despite high conservation of the selectivity filter, the K(+) transport properties of different K channels vary widely, with the unitary conductance spanning a range of over two orders of magnitude. Mutation of Pro475, a residue located at the cytoplasmic entrance of the pore of the small-intermediate conductance K channel Shaker (Pro475Asp (P475D) or Pro475Gln (P475Q)), increases Shaker's reported ∼ 20-pS conductance by approximately six- and approximately threefold, respectively, without any detectable effect on its selectivity. These findings suggest that the structural determinants underlying the diversity of K channel conductance are distinct from the selectivity filter, making P475D and P475Q excellent probes to identify key determinants of the K channel unitary conductance. By measuring diffusion-limited unitary outward currents after unilateral addition of 2 M sucrose to the internal solution to increase its viscosity, we estimated a pore internal radius of capture of ∼ 0.82 Å for all three Shaker variants (wild type, P475D, and P475Q). This estimate is consistent with the internal entrance of the Kv1.2/2.1 structure if the effective radius of hydrated K(+) is set to ∼ 4 Å. Unilateral exposure to sucrose allowed us to estimate the internal and external access resistances together with that of the inner pore. We determined that Shaker resistance resides mainly in the inner cavity, whereas only ∼ 8% resides in the selectivity filter. To reduce the inner resistance, we introduced additional aspartate residues into the internal vestibule to favor ion occupancy. No aspartate addition raised the maximum unitary conductance, measured at saturating [K(+)], beyond that of P475D, suggesting an ∼ 200-pS conductance ceiling for Shaker. This value is approximately one third of the maximum conductance of the large conductance K (BK) channel (the K channel of highest conductance), reducing the energy gap between their K(+) transport rates to ∼ 1 kT. Thus, although Shaker's pore sustains ion translocation as the BK channel's does, higher energetic costs of ion stabilization or higher friction with the ion's rigid hydration cage in its narrower aqueous cavity may entail higher resistance.
authors.affiliation: Centro Interdisciplinario de Neurociencia de Valparaíso and Programa de Doctorado en Ciencias mención Neurociencia, Universidad de Valparaíso, Valparaíso 2360103, Chile Centro Interdisciplinario de Neurociencia de Valparaíso and Programa de Doctorado en Ciencias mención Neurociencia, Universidad de Valparaíso, Valparaíso 2360103, Chile.
authors.forename: Ignacio
authors.initials: I
authors.lastname: Díaz-Franulic
authors.affiliation: Center for Bioinformatics and Integrative Biology, Universidad Andrés Bello, Santiago 8370146, Chile.
authors.forename: Romina V
authors.initials: RV
authors.lastname: Sepúlveda
authors.affiliation: Centro Interdisciplinario de Neurociencia de Valparaíso and Programa de Doctorado en Ciencias mención Neurociencia, Universidad de Valparaíso, Valparaíso 2360103, Chile.
authors.forename: Nieves
authors.initials: N
authors.lastname: Navarro-Quezada
authors.affiliation: Centro Interdisciplinario de Neurociencia de Valparaíso and Programa de Doctorado en Ciencias mención Neurociencia, Universidad de Valparaíso, Valparaíso 2360103, Chile Center for Bioinformatics and Integrative Biology, Universidad Andrés Bello, Santiago 8370146, Chile.
authors.forename: Fernando
authors.initials: F
authors.lastname: González-Nilo
authors.affiliation: Centro Interdisciplinario de Neurociencia de Valparaíso and Programa de Doctorado en Ciencias mención Neurociencia, Universidad de Valparaíso, Valparaíso 2360103, Chile david.naranjo@uv.cl.
authors.forename: David
authors.initials: D
authors.lastname: Naranjo
mesh_terms: D000200:Action Potentials; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D015640:Ion Channel Gating; D008969:Molecular Sequence Data; D020125:Mutation, Missense; D011188:Potassium / Q000378:metabolism; D051637:Shaker Superfamily of Potassium Channels / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D014981:Xenopus
chemical_list: D051637:Shaker Superfamily of Potassium Channels; D011188:Potassium