The Desensitization Gating of the MthK K ⁺ Channel Is Governed by Its Cytoplasmic Amino Terminus

Abstract

The RCK-containing MthK channel undergoes two inactivation processes: activation-coupled desensitization and acid-induced inactivation. The acid inactivation is mediated by the C-terminal RCK domain assembly. Here, we report that the desensitization gating is governed by a desensitization domain (DD) of the cytoplasmic N-terminal 17 residues. Deletion of DD completely removes the desensitization, and the process can be fully restored by a synthetic DD peptide added in trans. Mutagenesis analyses reveal a sequence-specific determinant for desensitization within the initial hydrophobic segment of DD. Proton nuclear magnetic resonance ( ¹H NMR) spectroscopy analyses with synthetic peptides and isolated RCK show interactions between the two terminal domains. Additionally, we show that deletion of DD does not affect the acid-induced inactivation, indicating that the two inactivation processes are mutually independent. Our results demonstrate that the short N-terminal DD of MthK functions as a complete moveable module responsible for the desensitization. Its interaction with the C-terminal RCK domain may play a role in the gating process.

Author Summary

Nerve cells use ion channels, pores in the cell membrane, to send messages in the form of electrical signals between cells. Most ion channels have evolved several elaborate mechanisms that allow the channels to close quickly after opening to prevent wasteful leakage of the electrochemical potential—the currency of neuron communication—across the cell membrane. The process is known as inactivation or desensitization. Previous study on the model RCK-containing MthK K ⁺ channel in the enlarged Escherichia coli membrane has shown that this archaeon channel also undergoes desensitization. Using the same method, we demonstrate that the desensitization is indeed an intrinsic molecular property of the MthK protein. We show that a specific region of MthK, the short N terminus of the protein, functions as a structurally independent domain and is entirely responsible for the desensitization gating process. Moreover, we show that this N-terminal domain interacts with the C-terminal RCK domain as part of the desensitization mechanism. This unique desensitization mechanism, by interaction between the two cytoplasmic termini, is distinct from those traditional mechanisms known as N- and C-type inactivation found in many voltage-gated Na ⁺ and K ⁺ channels or as the desensitization observed in the glutamate receptors. Since the KTN/RCK domain is found in a large number of prokaryotic K ⁺ channels and transporters, this unique mechanism may be common to these transport systems for regulating the K ⁺ flux through the cell membrane.

Abstract

The N terminus of the ion channel MthK functions as a structurally independent domain and is entirely responsible for the desensitization gating process required for neuron-to-neuron communication.