Kirkwood A, Weiner D, Lisman J E
Department of Biology, Brandeis University, Waltham, MA 02254.
Proc Natl Acad Sci U S A. 1989 May;86(10):3872-6. doi: 10.1073/pnas.86.10.3872.
Previous work by others on Limulus photoreceptors has shown that application of a variety of guanine nucleotide-binding regulatory protein (G protein) activators produces discrete waves of depolarization similar to those generated by single photos, but smaller in size. We investigated whether these events might originate at a site other than the G protein. Initiation of the events did not depend on the state of the visual pigment, suggesting that the events do not originate at the pigment level. The events could be blocked by the G-protein blocker guanosine 5'-[beta-thio]diphosphate (GDP[betaS]) and thus support the conclusion that these discrete events are due to the activation of G protein itself. Quantitative measurements indicate that the average size of these events is approximately 8 times smaller than that evoked by single photons under the same conditions. Given certain reasonable assumptions, these results imply that the gain of the first stage of transduction in vivo is approximately 8, a value considerably lower than that measured in vitro in vertebrate rods (gain, 100-500). Furthermore, independent evidence for a low first-stage gain in Limulus is derived from the observation that GDP[betaS] barely affects the size of the response to single photons, but greatly reduces the probability that a photon evokes a response. These results can be explained if rhodopsin normally activates only a few G proteins.
其他人之前对鲎光感受器的研究表明，应用多种鸟嘌呤核苷酸结合调节蛋白（G蛋白）激活剂会产生离散的去极化波，类似于单个光产生的去极化波，但尺寸较小。我们研究了这些事件是否可能起源于G蛋白以外的位点。这些事件的起始并不取决于视觉色素的状态，这表明这些事件并非起源于色素水平。这些事件可被G蛋白阻滞剂鸟苷5'-[β-硫代]二磷酸（GDP[βS]）阻断，因此支持了这些离散事件是由于G蛋白本身被激活的结论。定量测量表明，在相同条件下，这些事件的平均大小比单个光子诱发的事件小约8倍。基于某些合理的假设，这些结果意味着体内转导第一阶段的增益约为8，该值远低于在脊椎动物视杆细胞中体外测量的值（增益为100 - 500）。此外，鲎第一阶段增益较低的独立证据来自于以下观察结果：GDP[βS]几乎不影响对单个光子的反应大小，但大大降低了光子引发反应的概率。如果视紫红质通常仅激活少数G蛋白，那么这些结果就可以得到解释。
