Counting Ion and Water Molecules in a Streaming File through the Open-Filter Structure of the K Channel

Abstract

The mechanisms underlying the selective permeation of ions through channel molecules are a fundamental issue related to understanding how neurons exert their functions. The "knock-on" mechanism, in which multiple ions in the selectivity filter are hit by an incoming ion, is one of the leading concepts. This mechanism has been supported by crystallographic studies that demonstrated ion distribution in the structure of the Streptomyces lividans (KcsA) potassium channel. These still pictures under equilibrium conditions, however, do not provide a snapshot of the actual, ongoing permeation processes. To understand the dynamics of permeation, we determined the ratio of the ion and water flow [the water-ion coupling ratio (CR w-i )] through the KcsA channel by measuring the streaming potential (V stream ) electrophysiologically. The V stream value was converted to the CR w-i value, which reveals how individual ion and water molecules are queued in the narrow and short filter during permeation. At high K ϩ concentrations, the CR w-i value was 1.0, indicating that turnover between the alternating ion and water arrays occurs in a single-file manner. At low K ϩ , the CR w-i value was increased to a point over 2.2, suggesting that the filter contained mostly one ion at a time. These average behaviors of permeation were kinetically analyzed for a more detailed understanding of the permeation process. Here, we envisioned the permeation as queues of ion and water molecules and sequential transitions between different patterns of arrays. Under physiological conditions, we predicted that the knock-on mechanism may not be predominant.

Role of methyl-induced polarization in ion binding

“…In addition, electrophysiological studies show that T→S substitutions in Kir 2.1 alter the channel's mean open probabilities (16). The binding of K + ions to the S4 site also plays a vital role in the concerted movement of ions along the permeation pathway (10,(45)(46)(47)(48)(49)(50)(51)(52). A T→S substitution could therefore potentially also alter the kinetics of K + transport through the channel, although no such effects were observed in experiments on Kcv and Kir 2.1 channels (16).…”

Ion permeation in K + channels occurs by direct Coulomb knock-on

“…This is seemingly in conflict with the ion/water cotranslocation ratio derived from measurements of water translocation through KcsA (32)(33)(34)(35). However, these experiments were based on the application of high osmotic gradients.…”

Lipid Bilayers Manipulated through Monolayer Technologies for Studies of Channel-Membrane Interplay

“…In parallel to the patch-clamp methods, many methods using glass pipettes have been developed for planar lipid bilayer experiments, such as the tip-dip, [35][36][37][38] punch-out, 40) bubble patch, 48) liposome patch 49,50) and modified tip-dip 32) methods (Fig. 5).…”