University of Science and Technology of China reveals the structural basis of KAT1 hyperpolarization-activated inward rectification of the potassium ion channel that regulates stomatal pore changes in plant leaves
Ion channels are a kind of hydrophobic membrane proteins with hydrophilic pores on the cell membrane, which can selectively permeate different ions (e.g. K+, Na+, Ca2+, Cl-, etc.). It can selectively permeate different ions (e.g. K+, Na+, Ca2+, Cl-, etc.). The opening and closing of the pores of voltage-gated ion channels are regulated by the membrane potential on both sides of the cell membrane. Different ion channels are affected by the membrane potential in different ways, including depolarization (depolarization where the intramembrane voltage is higher than the extramembrane voltage) or hyperpolarization (hyperpolarization where the extramembrane voltage is higher than the intramembrane voltage).
Potassium ions are key elements in plant life and play an essential role in plant growth, development and various physiological processes. KAT1 is a potassium ion channel expressed in stomatal guard cells of Arabidopsis leaves. It has been shown that KAT1 is a voltage-gated inward rectifying potassium channel that mediates K+ inward flow, causing stomatal expansion and opening, and plays a key role in regulating stomatal pore changes on the leaf surface of Arabidopsis.
KAT1 belongs to a relatively rare class of hyperpolarization-activated-inward rectifying K+ channels. In contrast to typical depolarization-activated K+ channels, KAT1 has a unique reverse voltage dependence: depolarization leads to channel closure and hyperpolarization leads to channel opening and opening of K+ to inward flow (voltage gated K+ channels in higher animals are usually outward rectifying K+ ion channels). Currently, hyperpolarization-activated inward rectifying ion channels are very poorly studied, and the structural mechanisms underlying their unique gating properties remain poorly understood.
On September 8, 2020, Changlin Tian's group at the University of Science and Technology of China (USTC) published a paper entitled "Cryo-EM structure of the hyperpolarization-activated inwardly rectifying potassium channel KAT1 from Arabidopsis",
which reports the cryoelectron microscopic structure of KAT1, a hyperpolarized inwardly rectifying potassium channel from Arabidopsis, with an overall resolution of 3.2 Å. The structure of KAT1 is the first reported potassium channel of plant origin. ion channel structure.
The KAT1 cryoelectron microscopy structure indicates that KAT1 has a typical "non-domain-swapped" topology, which is similar to that of animal-derived Eag (Kv10.1), hErg (Kv11.1), HCN1 and other channels with higher structure. KAT1 has a K+ ion selectivity filter consisting of amino acids such as Thr-Thr-Gly-Tyr-Gly and a K+ ion selectivity filter consisting of amino acids such as (RIL)(SML)(RLW)(RLR)(RVS), and a K+ ion selectivity filter consisting of amino acids such as (RIL)(SML)(RLW)(RLR)(RVS). The voltage-receptor (voltage sensor) composed of amino acids is consistent with the classical structural features of potassium ion channels. A structural comparison with ion channels such as Kv1.2, Eag, hErg and HCN1 reveals that the S4 transmembrane helix itself as a voltage sensor does not determine the hyperpolarization activation of KAT1.
The combination of the membrane clamp electrophysiology of KAT1 and the structural data of KAT1 cryoelectron microscopy indicates that there is a more stable interaction between the linker structure between the S4, S5 transmembrane helix of KAT1 and the C-linker structure at the posterior end of the S6 helix, and that under hyperpolarizing conditions, the S4 helix slides into the cell membrane and pushes the S6 helix, thus opening the channel. This work reveals the structural similarities as well as differences between KAT1 and classical potassium ion channels, providing new clues to elucidate the gating mechanism of ion channels, especially the physicochemical mechanism of hyperpolarization-activated inward rectifying K+ channels.
