Paddle-chimaera K+ channel
Some membrane proteins detect membrane potential, the potential difference between extracellular and intracellular regions, and change their structure accordingly. These are, for example, voltage-dependent ion channels, voltage-dependent proton channels, and voltage-regulated enzymes, among which major amino acids of the voltage sensor are conserved. The voltage sensor is a membrane protein domain with charged amino acids and its voltage-dependent conformational change leads to a gating of the pore region.
A crystal structure of a paddle-chimaera K+ channel, Kv2.1 channel with a voltage-sensor paddle of Kv1.2, has been determined (Fig. 1, 2). This structure shows an open conformation and four potassium ions are observed at the center of the paddle-chimaera channel (Fig. 2). The voltage-sensor paddle is a helix-turn-helix structural motif composed of S3b and S4 helices. Voltage sensors are molecular volt meters that read the membrane voltage and regulate the pore. Generally, potassium ion channels form tetramers in which a pore region is centered with potassium ions and four voltage sensors surround the pore. The paddle-chimaera channel also showed good similarity with other K+ channels. Additionally, this crystal structure was constructed under a mixture of phospholipids and detergents to provide a membrane-like environment. Lipid molecules form bilayer-like arrangement, especially in the outer leaflet part (Fig. 3).
There are many charged residues in the voltage sensors which are embedded in hydrophobic cell membranes (Fig. 4). Negatively charged amino acids in the voltage sensor are divided into two clusters: an external negative cluster consisting of glutamate 183 and 226, and an internal negative cluster consisting of glutamate 154, 236, and aspartate 259. On the other hand, positively charged amino acids are aligned on the S4 helix (R0, R1, R2, R3, R4, K5 and R6, from extracellular to intracellular regions). R3 and R4 form hydrogen bonds with the external negative cluster. Besides, phenylalanine 233 is one of most conserved residues and separates negative clusters (Fig. 4).
A hypothesis for the gating mechanism was proposed. Voltage sensor paddles switches their interaction partners between the two negative clusters depends on the membrane voltage. The large paddle movement across the membrane results in the charge of the pore structure. However it should be noted that how the paddles overcome an energy barrier at the phenylalanine 233 is still unknown. So, a closed structure is needed to reveal new interactions on the S4 helix.
Protein Data Bank (PDB)
Long, S.B. Tao, X. Campbell, E.B. MacKinnon, R.; "Atomic structure of a voltage-dependent K+ channel in a lipid membrane-like environment."; Nature; (2007) 450:376-382 PubMed:18004376.
author: Naoya Fujita