MOLECULAR-IDENTIFICATION OF A HYPERPOLARIZATION-ACTIVATED CHANNEL IN SEA-URCHIN SPERM

Sea urchin eggs attract sperm through chemotactic peptides, which evok e complex changes in membrane voltage and in the concentrations of cyc lic AMP, cyclic GMP and Ca2+ ions (see ref. 1 for a review). The intra cellular signalling pathways and their cellular targets are largely un known. We have now cloned, from sea urchin testis, the complementary D NA encoding a channel polypeptide, SPIH, Functional expression of SPIH gives rise to weakly K+-selective hyperpolarization-activated channel s, whose activity is enhanced by the direct action of cAMP. Thus, SPIH is under the dual control of voltage and cAMP. The SPIH channel, whic h is confined to the sperm flagellum, may be involved in the control o f flagellar beating. SPM currents exhibit all the hallmarks of hyperpo larization-activated currents (I-h)(2,3), which participate in the rhy thmic firing of central neurons, control pacemaking in the heart, and curtail saturation by bright light in retinal photoreceptors(2,3). Bec ause of their sequence(4) and functional properties, I-h channels form a class of their own within the superfamily of voltage-gated and cycl ic-nucleotide-gated channels.