Outward Rectification of Voltage-Gated K⁺ Channels Evolved at Least Twice in Life History

<div><p>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 (K_in) and depolarization-activated, outward-rectifying (K_out) 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 K_in and K_out 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 K_in and K_out channels. Among them is a loop between TMD S5 and the pore that is very short in animal K_out, longer in plant and animal K_in and the longest in plant K_out 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.</p>