Braun F J, Hegemann P
Institut für Biochemie I, Universität Regensburg, 93040 Regensburg, Germany.
Biophys J. 1999 Mar;76(3):1668-78. doi: 10.1016/S0006-3495(99)77326-1.
Photoreceptor currents of the multicellular green alga Volvox carteri were analyzed using a dissolver mutant. The photocurrents are restricted to the eyespot region of somatic cells. Photocurrents are detectable from intact cells and excised eyes. The rhodopsin action spectrum suggests that the currents are induced by Volvox rhodopsin. Flash-induced photocurrents are a composition of a fast Ca2+-carried current (PF) and a slower current (PS), which is carried by H+. PF is a high-intensity response that appears with a delay of less than 50 micros after flash. The stimulus-response curve of its initial rise is fit by a single exponential and parallels the rhodopsin bleaching. These two observations suggest that the responsible channel is closely connected to the rhodopsin, both forming a tight complex. At low flash energies PS is dominating. The current delay increases up to 10 ms, and the PS amplitude saturates when only a few percent of the rhodopsin is bleached. The data are in favor of a second signaling system, which includes a signal transducer mediating between rhodopsin and the channel. We present a model of how different modes of signal transduction are accomplished in this alga under different light conditions.
利用一种溶解突变体分析了多细胞绿藻卡特氏团藻的光感受器电流。光电流局限于体细胞的眼点区域。完整细胞和分离出的眼点均可检测到光电流。视紫红质作用光谱表明电流是由团藻视紫红质诱导产生的。闪光诱导的光电流由快速的Ca2+携带电流（PF）和较慢的电流（PS）组成，PS由H+携带。PF是一种高强度反应，在闪光后不到50微秒出现延迟。其初始上升的刺激-反应曲线符合单指数曲线，且与视紫红质漂白平行。这两个观察结果表明，相关通道与视紫红质紧密相连，二者形成紧密复合物。在低闪光能量下，PS占主导。电流延迟增加至10毫秒，当只有百分之几的视紫红质被漂白时，PS幅度达到饱和。数据支持第二种信号系统，该系统包括一种在视紫红质和通道之间起介导作用的信号转导器。我们提出了一个模型，说明在不同光照条件下这种藻类如何完成不同的信号转导模式。
