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Sci. Signal., 27 October 2009
Vol. 2, Issue 94, p. ra68
Finding an Egg in an Ocean
Sperm of the sea urchin Arbacia punctulata, which are released into the ocean and must find their way to an egg before fertilization can take place, can sense and respond to a single molecule of the egg-derived chemoattractant resact. This response depends on the production of guanosine 3',5'-monophosphate (cGMP) and the consequent activation of K+-selective cyclic nucleotide–gated (CNGK) channels, which leads to production of an intracellular calcium signal that regulates movement of the sperm flagellum and thereby the direction in which the sperm cell swims. After cloning the A. punctulata CNGK, Bönigk et al. combined mutational analysis with optical analysis and electrophysiology to explore the mechanisms responsible for this sensitivity. They found that, although CNGK contains four repeating regions, each of which resembles a cyclic nucleotide–gated (CNG) channel subunit and contains a cyclic nucleotide–binding domain, it is activated through binding of only a single molecule of cGMP. Using a compound that cages cGMP and becomes fluorescent after its release, they were able to calibrate the system and determine that fewer than 50 molecules of cGMP were required to mediate the Ca2+ response to a single molecule of resact.
Citation: W. Bönigk, A. Loogen, R. Seifert, N. Kashikar, C. Klemm, E. Krause, V. Hagen, E. Kremmer, T. Strünker, U. B. Kaupp, An Atypical CNG Channel Activated by a Single cGMP Molecule Controls Sperm Chemotaxis. Sci. Signal.2, ra68 (2009).