Kcv, the first identified viral potassium channel encoded by the green algae Paramecium bursaria chlorella virus (PBCV-1), conducted K+ selective currents when expressed in heterologous systems. This K+ channel was proposed to be important for PBCV-1 infection and replication. In the present study, we identified and functionally characterized a novel K+ channel Kesv, encoded by Ectocarpus siliculosus virus that infects filamentous marine brown algae. Kesv encodes a protein of 124 amino acids and is 21.8% identical and 37.1% homologous to Kcv. Membrane topology programs predicted that Kesv consists of three transmembrane domains. When expressed in Xenopus oocytes, Kesv induced largely instantaneous, K+ selective currents that were sensitive to block by Ba2+ and amantadine. Thus, Kesv along with Kcv, constitutes an emerging family of viral potassium channels, which may play important roles in the life cycle of viruses. (C) 2004 Elsevier Inc. All rights reserved.