| Summary: | Potassium ion (K<sup>+</sup>) channels have been observed in diverse viruses that infect eukaryotic marine and freshwater algae. However, experimental evidence for functional K<sup>+</sup> channels among these alga-infecting viruses has thus far been restricted to members of the family <i>Phycodnaviridae,</i> which are large, double-stranded DNA viruses within the phylum <i>Nucleocytoviricota</i>. Recent sequencing projects revealed that alga-infecting members of <i>Mimiviridae</i>, another family within this phylum, may also contain genes encoding K<sup>+</sup> channels. Here we examine the structural features and the functional properties of putative K<sup>+</sup> channels from four cultivated members of <i>Mimiviridae</i>. While all four proteins contain variations of the conserved selectivity filter sequence of K<sup>+</sup> channels, structural prediction algorithms suggest that only two of them have the required number and position of two transmembrane domains that are present in all K<sup>+</sup> channels. After in vitro translation and reconstitution of the four proteins in planar lipid bilayers, we confirmed that one of them, a 79 amino acid protein from the virus Tetraselmis virus 1 (TetV-1), forms a functional ion channel with a distinct selectivity for K<sup>+</sup> over Na<sup>+</sup> and a sensitivity to Ba<sup>2+</sup>. Thus, virus-encoded K<sup>+</sup> channels are not limited to <i>Phycodnaviridae</i> but also occur in the members of <i>Mimiviridae</i>. The large sequence diversity among the viral K<sup>+</sup> channels implies multiple events of lateral gene transfer.
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