{'fraction_non_alphanumeric': 0.021869488536155203, 'fraction_numerical': 0.003527336860670194, 'mean_word_length': 5.3162583518930955, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
An Atypical CNG Channel Activated by a Single cGMP Molecule Controls Sperm Chemotaxis

The ability of a single molecule of cGMP to activate the K+-selective cyclic nucleotide–gated channel allows sea urchin sperm to find an egg. 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. Sperm of the sea urchin Arbacia punctulata can respond to a single molecule of chemoattractant released by an egg. The mechanism underlying this extreme sensitivity is unknown. Crucial signaling events in the response of A. punctulata sperm to chemoattractant include the rapid synthesis of the intracellular messenger guanosine 3′,5′-monophosphate (cGMP) and the ensuing membrane hyperpolarization that results from the opening of potassium–selective cyclic nucleotide–gated (CNGK) channels. Here, we use calibrated photolysis of caged cGMP to show that ~45 cGMP molecules are generated during the response to a single molecule of chemoattractant. The CNGK channel can respond to such small cGMP changes because it is exquisitely sensitive to cGMP and activated in a noncooperative fashion. Like voltage-activated Cav and Nav channels, the CNGK polypeptide consists of four homologous repeat sequences. Disabling each of the four cyclic nucleotide–binding sites through mutagenesis revealed that binding of a single cGMP molecule to repeat 3 is necessary and sufficient to activate the CNGK channel. Thus, CNGK has developed a mechanism of activation that is different from the activation of other CNG channels, which requires the cooperative binding of several ligands and operates in the micromolar rather than the nanomolar range.