We offer KcsA, potassium channel K channel of streptomyces A, Ion Channel, cell-free protein isolated by nanodiscs, based on E.coli lysates for cell-free expression.

Our CELL-FREE protein has been Function verificated.

Introduction

KcsA (K channel of streptomyces A) is a prokaryotic potassium channel from the soil bacteria Streptomyces lividans that has been studied extensively in ion channel research. The pH activated protein possesses two transmembrane segments and a highly selective pore region, responsible for the gating and shuttling of K⁺ ions out of the cell. The amino acid sequence found in the selectivity filter of KcsA is highly conserved among both prokaryotic and eukaryotic K⁺ voltage channels; as a result, research on KcsA has provided important structural and mechanistic insight on the molecular basis for K⁺ ion selection and conduction. As one of the most studied ion channels to this day, KcsA is a template for research on K⁺ channel function and its elucidated structure underlies computational modeling of channel dynamics for both prokaryotic and eukaryotic species.

Function

The KcsA channel is considered a model channel because the KcsA structure provides a framework for understanding K⁺ channel conduction, which has three parts: Potassium selectivity, channel gating by pH sensitivity, and voltage-gated channel inactivation. K⁺ ion permeation occurs at the upper selectivity filter region of the pore, while pH gating rises from the protonation of transmembrane helices at the end of the pore. At low pH, the M2 helix is protonated, shifting the ion channel from closed to open conformation. As ions flow through the channel, voltage gating mechanisms are thought to induce interactions between Glu71 and Asp80 in the selectivity filter, which destabilize the conductive conformation and facilitate entry into a long-lived nonconducting state that resembles the C-type–inactivation of voltage-dependent channels.

In the nonconducting conformation of KcsA at pH 7, K⁺ is bound tightly to coordinating oxygens of the selectivity filter and the four TM2 helices converge near the cytoplasmic junction to block the passage of any potassium ions. At pH 4 however, KcsA undergoes millisecond-timescale conformational exchanges filter permeating and nonpermeating states and between the open and closed conformations of the M2 helices. While these distinct conformational changes occur in separate regions of the channel, the molecular behavior of each region is linked by both electrostatic interactions and allostery. The dynamics of this exchange stereochemical configurations in the filter provides the physical basis for simultaneous K⁺ conductance and gating.

Stucture

With the emergence of the crystal structure (0.32 nm) of the bacterial (Streptomyces lividans) potassium channel KcsA, a keystone for ion channel structure has been delivered. Its topology has been identified to be bitopic with an inner and outer helix. The two helices are linked by the “pore helix” and the selectivity filter. Four subunits form the channel by lining a pathway for ions to cross the bilayer. The passage of an ion can be described as passing a gate on the cytoplasmic side and moving into a vestibule which stabilizes the ion via the dipole moment of a so-called inner helix. The ions need to pass the selectivity filer in a concerted motion with already potassium ions located within the filter already. With its high selectivity, KcsA shows similarity with voltage-gated K channels (Kv) in terms of ion permeation and similarity in topology with inward rectifying K channels . Important to note is that KcsA channels are triggered by pH changes. Deciphering the cause of the enormous selectivity of K channels for potassium over sodium is a challenge up to date. It is anticipated that the structure of KcsA represents a closed stage. With the structure (0.33 nm) of a prokaryotic (Methanobacterium thermoautotrophicum). During activation, the inner helices push radial outward and open the constriction at the cytoplasmic side. This opening is accompanied by a hinge around a glycine residue at position 99 (G99). This is the most pronounced conformational change so far described for ion channels. The selectivity filter remains unchanged in its structural position and the outer helix only undergoes marginal conformational changes. As a conclusion for the context of this review and in respect to the findings for Vpu in relation to KcsA.

Advantage of E.Coli system

1. High protein yield
2. Simple cultivation and fast cell growth and lysate preparation
3. Cost-efficient
4. Easy genetic engineering
5. Well-established