Voltage-gated potassium (K_v) channels work in concert with other ion channels to determine the frequency and duration of action potentials in excitable cells. Little is known about K_v3 channels from invertebrates, but those that have been characterized generally display slow kinetics. Here, we report the cloning and characterization of jShaw1, the first K_v3 isolated from a cnidarian, the jellyfish Polyorchis penicillatus, in comparison with mouse K_v3.1 and K_v3.2. Using a two-electrode voltage clamp on Xenopus laevis oocytes expressing the channels, we compared steady-state and kinetic properties of macroscopic currents. jShaw1 is fast activating, and opens at potentials approximately 40 mV more hyperpolarized than the mouse K_v3 channels. There is an inverse relationship between the number of positive charges on the voltage sensor and the half-activation voltage of the channel, contrary to what would be expected with the simplest model of voltage sensitivity. jShaw1 has kinetic characteristics that are substantially different from the mammalian K_v3 channels, including a much lower sensitivity of early activation rates to incremental voltage changes, and a much faster voltage-dependent transition in the last stages of opening. jShaw1 opening kinetics were affected little by pre-depolarization voltage, in contrast to both mouse channels. Similar to the mouse channels, jShaw1 was half-blocked by 0.7 mmol l^–1 tetraethyl ammonium and 5 mmol l^–1 4-aminopyridine. Comparison of sequence and functional properties of jShaw1 with the mouse and other reported K_v3 channels helps to illuminate the general relationship between amino acid sequence and electrophysiological activity in this channel family.

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