Abstract
Voltage-gated potassium (K+) channels are present in all living systems. Despite high structural similarities in the transmembrane domains (TMD), this K+ channel type segregates into at least two main functional categories—hyperpolarization-activated, inward-rectifying (Kin) and depolarization-activated, outward-rectifying (Kout) channels. Voltage-gated K+ channels sense the membrane voltage via a voltage-sensing domain that is connected to the conduction pathway of the channel. It has been shown that the voltage-sensing mechanism is the same in Kin and Kout channels, but its performance results in opposite pore conformations. It is not known how the different coupling of voltage-sensor and pore is implemented. Here, we studied sequence and structural data of voltage-gated K+ channels from animals and plants with emphasis on the property of opposite rectification. We identified structural hotspots that alone allow already the distinction between Kin and Kout channels. Among them is a loop between TMD S5 and the pore that is very short in animal Kout, longer in plant and animal Kin and the longest in plant Kout channels. In combination with further structural and phylogenetic analyses this finding suggests that outward-rectification evolved twice and independently in the animal and plant kingdom.
Highlights
Voltage-gated potassium (K+) channels have been investigated in deep detail in various organisms ranging from pro- to eukaryotic species
For our comparison of voltage-gated potassium (K+) channels from plants and animals we used as prototypes the nine Kv-like channels from the model plant Arabidopsis thaliana, 12 hyperpolarization-activated cyclic nucleotide-gated cation channels (HCNs) and 10 Kv channels from Rattus norvegicus, Mus musculus and Homo sapiens, as well as the HCN from the sea urchin, Strongylocentrotus purpuratus, and the Shaker channel from Drosophila melanogaster
According to their voltage-dependence, these channels were categorized into hyperpolarization-activated, inward-rectifying (Kin) and depolarization-activated, outward-rectifying (Kout) channels [12]. Based on their voltage-dependence and on their highest permeability for K+, HCN channels are considered as animal Kin channels they are no strict K+ channels and permeable to other cations, like e.g. Na+ [15]
Summary
Voltage-gated potassium (K+) channels have been investigated in deep detail in various organisms ranging from pro- to eukaryotic species (reviewed in [1,2,3,4]). Having a common subunit structure, these channels consist of six transmembrane domains (TMD), S1 to S6, and a pore helix and selectivity filter between the last two TMD (S5–P–S6) (Fig 1A). Functional channels are tetramers, in which the four subunits are twisted with each other. The S5-P-S6 parts act together in the centre of the channel and form the ion conduction pathway. TMDs S1-S4 form the voltage sensors that are located in the periphery of the channel. They are connected to the pore via a short amino acid sequence, the S4-S5 linker
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