Mechanism of photoactivation of photoactivated adenylyl cyclase (PAC), a novel blue-light receptor, and mechanism of photocontrol of cellular functions.

光激活腺苷酸环化酶（PAC） （isiseki et al. 2002 Nature）是一种蓝光感应蛋白，负责真核单细胞鞭毛藻的避光行为。它由黄素结合、光感应结构域和腺苷酸环化酶催化结构域组成，因此在蓝光照射下产生多功能细胞内第二信使cAMP。通过对各种拟真藻类中PAC同源物的分子系统发育分析，表明PAC几乎是伴随着其叶绿体的次生共生引入而被引入真核藻类的（Koumura et al. 2004 Photochem Photobiol Sci）。通过光活化动力学分析，测定了纯化的绿枝草PAC的作用谱、对重复短脉冲辐照的响应以及细胞内cAMP浓度的时间过程。这些结果符合相应的体内光行为特征（Yoshikawa et al. 2005 Ph . More otochem Photobiol Sci）。通过分析黄素结合域在E. coil中异种表达的光化学循环动力学，确定了阈值光闪率、量子效率以及暗恢复器的半衰期，这些与所谓的黄素结合域BLUF家族的其他成员大致相似，主要是原核起源（Ito et al. 2005 Photochem Photobiol Sci）。PAC作为一种具有腺苷酸环化酶内在效应功能的光传感器的独特特征，为其在异种系统中成功表达的各种camp依赖的生物功能（包括神经和生殖过程）提供了有希望的可能性（isiseki et al. 2002 Nature）。事实上，这种可能性是通过PAC在青蛙卵母细胞、人类培养细胞以及苍蝇大脑中的成功功能表达实现的，特别是在外部光照射下，苍蝇个体的梳理行为被特异性地光抑制（Schroeder-Lang 2007 Nature Methods）。少

Photoactivated adenylyl cyclase (PAC) (Iseki et al. 2002 Nature) is a blue-light sensor protein responsible for photoavoidance behavior in a eukaryotic unicellular flagellate alga Euglena gracilis. It is composed of flavin-binding, light sensing domains and adenylyl cyclase catalytic domains so that it produces cAMP, a versatile intracellular second messenger, in response to blue-light irradiation.By molecular phylogenetic analyses of homologues fo PAC in various euglenoids, it was suggested that PAC was introduced into Euglena almost concomitantly with the secondary symbiotic introduction of its chroloplasts (Koumura et al. 2004 Photochem Photobiol Sci).By analyses of the photoactivation kinetics of purified PAC from Euglena, action spectrum and responsively to repeated short pulse irradiations were determined as well as intracellular cAMP concentration time courses. These results were in accordance with the corresponding in vivo photobehavior characteristics (Yoshikawa et al. 2005 Ph … More otochem Photobiol Sci).By analyses of the photochemical cycle kinetics of a flavin-binding domain heterologously expressed in E. coil, threshold light flounce rate, quantum efficiency as well as half life of dark recovery ware determined which were in general similarity to other members of the so-called BLUF family of flavin-binding domains, mostly of prokaryotic origins (Ito et al. 2005 Photochem Photobiol Sci).The unique distinguishing feature of PAC to act as a light sensor having intrinsic effector function of adenylyl cyclase provide promising possibilities to be used as a tool to photomanipulate various cAMP-dependent biological functions including neural and reproductive processes if it is successfully expressed functionally in heterologous systems (Iseki et al. 2002 Nature). This possibility was in fact realized by successful functional expression of PAC in frog oocytes, human cultured cells as well as in fly brains, particularly in which grooming behavior of the fly individual was specifically photosuppressed by external light irradiation (Schroeder-Lang 2007 Nature Methods). Less