An extravagant ion channel

The detection of chemical cues in the environment – which provide information on food, mates, danger, predators and pathogens – is essential for the survival of most animals. Not only whole organisms, but also single cells respond to such “chemical signals”. Even extremely low concentrations of a chemical stimulus can elicit a cellular response. A particular sensitive process endows organisms or cells to navigate in a gradient of chemical substances – called chemotaxis. Chemotaxis is important for reproduction in many species. The oocyte attracts sperm by releasing chemical substances, called chemoattractants. Sea urchins have served as model organism for sperm chemotaxis since the early 20th century. Once diluted in the vast ocean, oocytes and sperm do not meet accidentally.

Sperm of the sea urchin Arbacia punctulata respond exquisitely sensitive: they are able to detect a single molecule of chemoattractant. The mechanisms underlying this absolute sensitivity at the physical limit are not known. In the November issue of Science Signaling 2009, we report that an atypical cyclic nucleotide-gated (CNG) ion channel endows sperm with single-molecule sensitivity. The new CNG channel reveals unexpected functional and structure features. Classic CNG channels in retinal photoreceptors and olfactory neurons comprise four similar polypeptides, referred to as A and B subunits, that associate to form a central pore, i.e. they exist as tetramers. In contrast, the atypical CNG is formed from a single large polypeptide that carries four similar domains, referred to as “repeat”. Each of these repeats represents a subunit. The four repeats assemble into a pseudo-tetrameric fashion to form the central pore. The CNGK channel is a chimera that carries structural features from three different families of ion channels. Like classical CNG channels, it carries four cyclic nucleotide binding domains (CNBD). It has in the pore region the canonical sequence motif of voltage-dependent Kv channels. We, therefore, called this ion channel CNGK. Finally, the four-repeat structure is reminiscent of voltage-activated Nav and Cav channels.

While classical CNG channels are activated by cooperative binding of at least two cAMP or cGMP molecules, the CNGK opens after the binding of a single ligand molecule. Disabling each of the four CNBDs by mutagenesis revealed that CNBD3 is sufficient and necessary to open the CNGK channel. The CNBDs 1, 2, and 4 either do not bind ligands or the binding event does not open the channel. However, one of the most surprising properties of CNGK is its high ligand sensitivity in the nanomolar range K1/2 = 25 nM. A single activated chemosensory receptor produces about 20 – 45 cGMP molecules per second. This is equivalent to a change in the total cGMP concentration in the flagellum of about 20 – 45 nM. It is unlikely that each of the synthesized cGMP molecules activates a CNGK channel, because high-affinity buffers for cGMP and hydrolysis of cGMP by phosphodiesterase compete with CNGK for cGMP. Thus, the opening of a few CNGK channels may be sufficient to electrically excite sperm and evoke a Ca2+ signal. When only few cGMP molecules are available, the cooperative activation would impair rather than enhance channel sensitivity. Thus, classic CNG channels and CNGK have evolved different activation mechanisms that represent adaptations to conditions of high and low ligand concentrations.

Signaling of many neuropeptides, hormones, and pheromones involves high-affinity receptors in small cellular compartments such as synaptic boutons or sensory cilia. Thus, other signaling pathways may depend on single-molecule sensitivity as well.

Bönigk, W., Loogen, A., Seifert, R., Kashikar, N., Klemm, C., Krause, E., Hagen, V., Kremmer, E., Strünker, T., and Kaupp, U.B. (2009) "An Atypical CNG Channel Activated by a Single cGMP Molecule Controls Sperm Chemotaxis" Sci. Signal. 2, ra68