Potassium channels, integral membrane proteins that function as tetramers, create aqueous pores allowing the passage of potassium ions (K⁺). This fundamental structure is present in nearly all living organisms, from humans and fruit flies to Caenorhabditis elegans. Genomic studies reveal that these channels are represented by 30 to 100 distinct genes across various species. Insights into the gating mechanisms and ion flow of these channels have been achieved through the structural analysis of bacterial potassium channels, sequence comparisons, and electrophysiological measurements.

Discovery and Evolution of Potassium Channels

Potassium channels were first identified as molecular entities mediating potassium ion movement in neural membranes, crucial for action potential generation. However, research has expanded our understanding of their role beyond neurons. These channels are now known to be ubiquitous, present in nearly all cell types, and involved in a variety of physiological processes. They form tetrameric structures that create selective aqueous pores for K⁺. Potassium channels are categorized into different molecular subfamilies based on their response to physiological signals such as voltage, calcium ions (Ca²⁺), G proteins, and polyamines.

The evolution of potassium channels remains a topic of significant speculation, yet their widespread presence across life forms suggests that they are an ancient protein family rather than a highly specialized neurogenic machinery. Genomes from all fully sequenced organisms, whether eukaryotic, bacterial, or archaeal, contain at least one potassium channel gene. This widespread distribution underscores potassium channels as pioneers in ion channel evolution, primarily functioning in eukaryotes. Other ion channels in the S4 superfamily are predominantly eukaryotic.

Cyclin nucleotide-gated channels evolved from potassium channels by acquiring a cyclin nucleotide-binding domain near the C-terminus. Calcium (Ca²⁺) and sodium (Na⁺) channels evolved through two rounds of gene duplication from potassium channels. These evolutionary steps led to the formation of calcium and sodium channels in unicellular organisms and the emergence of neurons in multicellular organisms. As these specialized channels evolved, their pore structures adapted to accommodate different ion selectivities.

Figure 1. Membrane topologies and main features of the Kv and Kir potassium channel subtypes. (a, b) The schematic representation illustrates the membrane topology of (a) Kv channels and (b) Kir channels.

Organization and Evolution of Potassium Channel Genes

In humans, many potassium channel genes undergo complex splicing, though not all genes are affected. Current research does not yet provide a comprehensive overview of gene structure. In some cases, entire coding regions are located within a single exon, while in others, they are split across multiple exons, with some exons clearly defining functional domains. Potassium channel genes typically do not cluster on chromosomes.

The structural features of potassium channels fall into two main categories: six-transmembrane helix voltage-gated (Kv) and two-transmembrane helix inward rectifying (Kir). All potassium channels exhibit a characteristic sequence between the two transmembrane helices closest to the C-terminus, usually represented as TMxTVGYG (using single-letter amino acid codes). Additionally, Kv channels possess a unique S4 sequence in the fourth transmembrane segment (S4), where lysine or arginine appears in every third or fourth position within a largely hydrophobic stretch.

Despite variations in these fundamental sequences, such as an additional transmembrane segment near the N-terminus in the Ca²⁺-activated potassium channel "slowpoke" subfamily and a pair of tandem potassium channel sequence subunits in 2P channels, all potassium channel subunits share a core structure. This core includes two transmembrane helices separated by a rotating pore ring carrying characteristic sequences. These structures form a tetrameric assembly around a water-filled ion-conducting channel.

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Structural Analysis and Function of Potassium Channels

Current structural information about potassium channels primarily comes from a bacterial protein, KcsA, which belongs to the Kir topology type. Despite this, structural inferences from electrophysiological experiments on eukaryotic potassium channels are highly consistent with bacterial channel structures, allowing extrapolation of KcsA's features to the entire potassium channel family. Potassium channels consist of similar or identical subunits forming a tetrameric structure with four-fold symmetry. This is also true for 2P tandem channels. The common core structure of potassium channel subunits includes two transmembrane helices separated by a rotating pore ring carrying characteristic sequences. The helices closest to the C-terminus and the characteristic sequence together line most of the aqueous pore, determining the high K⁺ selectivity observed in most potassium channels.

The transmembrane span of the potassium channel aqueous pore exhibits significant asymmetry. The narrowest part of the pore is a 3 Å diameter selectivity filter, which emerges abruptly from the extracellular side and extends vertically through the membrane plane by about 10-15 Å. This structure's walls are non-charged but highly hydrophilic, composed mainly of 12 backbone carbonyl groups, with each subunit providing three. The pore then expands into a spherical hydration chamber with a diameter of 10 Å, situated in the membrane's middle. Although the chamber's walls are primarily composed of aliphatic side chains, they are inherently hydrophobic, similar to the pore at the intracellular end. In the KcsA structure, the pore is obstructed on the intracellular side by a "tent"-like structure formed by four transmembrane helices. This obstruction likely represents a closed state akin to the "gate" in Kv channels, with a conformational switch that can open or close the pore. In both Kv and Kir topologies, the amino and carboxyl termini are located on the cytoplasmic side of the membrane.

For six transmembrane Kv channels, the first four transmembrane segments (S1-S4) form a module responsible for controlling the opening and closing of the pore. The first three transmembrane segments are mostly helical and positioned at the lipid-exposed periphery of the membrane-embedded complex. The fourth transmembrane segment, S4, is not exposed to the lipid environment and is considered the voltage-sensing component of these channels. This S4 sequence is also found in some channels with minimal or no voltage-dependent gating, such as Ca²⁺-activated potassium channels and cyclic nucleotide-gated channels.

A conserved domain in Kv channels, known as the T1 domain, is located at the N-terminal side of the first transmembrane helix. The structure of the soluble isolated T1 domain from the California sea hare Kv1.1 channel has been elucidated. This domain exhibits a unique "hanging cable car" form in the full channel, where the tetrameric T1 domain aligns with the pore and is separated by approximately 20 Å from the membrane-embedded portion through protein chains that define the window for ions entering the cytoplasmic side of the pore, with the window width approximately 20 Å. Many Kv channels are associated with intracellular spherical "β-subunits," whose function remains unclear, but they bind to the lower part of the T1 domain.

Localization and Physiological Functions of Potassium Channels

Potassium channels are present in nearly all cell types and perform a variety of biological tasks, making it impossible to encompass all their functions comprehensively here. Nonetheless, all potassium channels share a fundamental function: creating a transmembrane "leak" channel highly specific to K⁺ ions. Given that cells generally maintain a much higher intracellular K⁺ concentration than extracellular, the opening of potassium channels leads to a negative change in the cell membrane voltage, known as "membrane hyperpolarization." This membrane hyperpolarization has different roles in various physiological contexts. For instance, in excitable cells like neurons, muscle cells, pancreatic cells, and cardiac myocytes, it terminates action potentials. In non-excitable cells like epithelial cells, endothelial cells, and lymphocytes, it drives the operation of other transmembrane ion channels or transport proteins.

While most potassium channels are voltage-gated, some respond to other physiological signals. Based on these signals, potassium channels can be classified into voltage-gated (Kv channels), inward-rectifying (Kir channels), calcium-activated (KCa channels), acid-sensing (K2P channels), and cyclin nucleotide-gated (HCN channels). These channels participate in numerous physiological processes through their specific gating mechanisms and ion selectivities, including heart rate regulation, muscle contraction, nerve conduction, and cell volume regulation.

Potassium channels are not only key components in neuronal electrical signal transmission but also play vital roles in virtually all cell types. Their intricate molecular structures and diverse gating mechanisms enable them to participate in a wide range of physiological functions, from gene expression regulation to cell signaling. Continued research into the structure-function relationships of potassium channels will further illuminate their critical roles in health and disease, providing valuable targets for novel therapeutic strategies.