Center of Advanced European Studies and Research, Abteilung Molekulare Neurosensorik, Ludwig-Erhard-Allee 2, 53175 Bonn, Germany.
Sci Signal. 2009 Oct 27;2(94):ra68. doi: 10.1126/scisignal.2000516.
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.
海胆精子可以对一个卵子释放的单个化学引诱物分子做出反应。这种极端敏感性的机制尚不清楚。在 A. punctulata 精子对化学引诱物的反应中，关键的信号事件包括细胞内信使鸟苷 3'，5'-环单磷酸（cGMP）的快速合成，以及随后由于钾选择性环核苷酸门控（CNGK）通道的打开而导致的膜超极化。在这里，我们使用被笼锁 cGMP 的精确光解来表明，在对单个化学引诱物分子的反应中，大约会产生 45 个 cGMP 分子。CNGK 通道可以对如此小的 cGMP 变化做出反应，因为它对 cGMP 非常敏感并且以非协同的方式被激活。与电压激活的 Ca(v)和 Na(v)通道一样，CNGK 多肽由四个同源重复序列组成。通过突变使四个环核苷酸结合位点失活表明，单个 cGMP 分子与重复 3 的结合对于激活 CNGK 通道是必要且充分的。因此，CNGK 已经开发出一种不同于其他 CNG 通道的激活机制，后者需要几个配体的协同结合，并且在微摩尔而不是纳摩尔范围内起作用。
