1. Howard Hughes Medical Institute, Laboratory of Molecular Neurobiology and Biophysics, Rockefeller University, 1230 York Avenue, New York, New York 10021, USA

Correspondence to: Roderick MacKinnon Correspondence and requests for materials should be addressed to R.M. (e-mail: Email: mackinn@rockvax.rockefeller.edu).

All living organisms use ion channels to regulate the transport of ions across cellular membranes¹. Certain ion channels are classed as voltage-dependent because they have a voltage-sensing structure that induces their pores to open in response to changes in the cell membrane voltage. Until recently, the voltage-dependent K⁺, Ca²⁺ and Na⁺ channels were regarded as a unique development of eukaryotic cells, adapted to accomplish specialized electrical signalling, as exemplified in neurons. Here we present the functional characterization of a voltage-dependent K⁺ (K_V) channel from a hyperthermophilic archaebacterium from an oceanic thermal vent. This channel possesses all the functional attributes of classical neuronal K_V channels. The conservation of function reflects structural conservation in the voltage sensor as revealed by specific, high-affinity interactions with tarantula venom toxins, which evolved to inhibit eukaryotic K_V channels.