An atypical CNG channel activated by a single cGMP molecule controls sperm chemotaxis.
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 approximately 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 Ca(v) and Na(v) 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.
Bönigk
Wolfgang
W
Center of Advanced European Studies and Research, Abteilung Molekulare Neurosensorik, Ludwig-Erhard-Allee 2, 53175 Bonn, Germany.
Loogen
Astrid
A
Seifert
Reinhard
R
Kashikar
Nachiket
N
Klemm
Clementine
C
Krause
Eberhard
E
Hagen
Volker
V
Kremmer
Elisabeth
E
Strünker
Timo
T
Kaupp
U Benjamin
UB
D000595:Amino Acid Sequence; D000818:Animals; D015153:Blotting, Western; D002633:Chemotaxis / Q000502:physiology*; D003001:Cloning, Molecular; D006152:Cyclic GMP / Q000502:physiology*; D054815:Cyclic Nucleotide-Gated Cation Channels / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D006868:Hydrolysis; D007150:Immunohistochemistry; D008297:Male; D008969:Molecular Sequence Data; D011499:Protein Processing, Post-Translational; D012617:Sea Urchins; D017386:Sequence Homology, Amino Acid; D013094:Spermatozoa / Q000166:cytology*
D054815:Cyclic Nucleotide-Gated Cation Channels; D006152:Cyclic GMP