An Atypical CNG Channel Activated by a Single cGMP Molecule Controls Sperm Chemotaxis

Abstract

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.

Structural insights into conformational changes of a cyclic nucleotide-binding domain in solution from Mesorhizobium loti K1 channel

“…Moreover, classic CNG and HCN channels are cooperatively activated by ligand or voltage, respectively. The allosteric mechanisms involve intersubunit contacts between long C linkers (4); the C-linker interface seems to be less important in MloK1 or CNGK channels that are activated noncooperatively (4,20,35). In contrast, in CAP, Epac, and PKA the signal is relayed to the DNA-binding domain or catalytic sites, respectively, that are located in the C terminus.…”

Calcium Channels in the Development, Maturation, and Function of Spermatozoa

“…As a consequence of intensive efforts in time-resolved fluorometry (286,351,387,388,485), Ca 2ϩ imaging (58, 227, 561-563), and molecular identification (60,195,211,223,486,487), our knowledge about the signaling cascade triggered by SAPs has advanced significantly in the last 10 years. FIGURE 8 depicts a proposed general scheme for SAP signaling.…”

The rate of change in Ca2+concentration controls sperm chemotaxis

“…Chemotactic signaling and behavior are most advanced in sperm of the sea urchin Arbacia punctulata. It involves binding of resact, the chemoattractant, to a receptor guanylyl cyclase, a rapid rise of the cellular messenger cyclic guanosine monophosphate (cGMP; Kaupp et al, 2003), a hyperpolarization as a result of the opening of K + -selective cyclic nucleotide-gated ion channels (Strünker et al, 2006;Galindo et al, 2007;Bönigk et al, 2009), and, finally, the opening of voltage-dependent Ca v channels. The periodic stimulation of sperm during circular swimming in a chemoattractant gradient entrains periodic Ca 2+ signals and alternating periods of high path curvature (turn) and low path curvature (run) that result in a looping swimming path toward the egg (Böhmer et al, 2005;Friedrich and Jülicher, 2007;Wood et al, 2007;Guerrero et al, 2010a,b).…”