Physiological and Biochemical Investigations on the Phototropins: Plant Blue Light Receptors

向光蛋白（phot 1和phot 2）是一类独特的植物光感受器家族，它们结合两个黄素单核苷酸（FMN）分子作为它们的发色团。蓝光和UV-A光引起这些光感受器的快速自磷酸化。在迄今为止研究的所有计划中，phot 1与质膜相关，初步证据表明phot 2也是如此。向光蛋白参与向光性（向蓝色或UV-A光源生长），叶绿体运动的光感知（在强光下通过相互遮蔽避免，在弱光下移动以最大化光拦截），气孔开放，快速抑制黑暗生长的幼苗的生长，以及在那些具有这种反应的物种中可能的太阳跟踪。FMN发色团紧密结合在指定为LOV 1和LOV 2的专门结构域（在对光、氧或电压作出反应的信号蛋白中发现的结构域）中。向光素LOV结构域经历独特的光化学，其涉及黄素三重态的光激活形成，黄素三重态在微秒内衰减至亚稳态信号传导状态，即FMN的C94 α）碳处的半胱氨酰加合物。这种信号状态在几秒或几分钟内衰减到黑暗状态，这取决于涉及哪个LOV结构域。该项目将探索这种相同的初始光化学（加合物形成）和生物化学（自磷酸化）如何激活这些不同的过程，其中一些需要细胞间的通信，而另一些则完全是细胞自主的。它还将探讨各个LOV域在这些不同过程中的作用。我们已经知道，只有LOV 2是phot 1完全自磷酸化所必需的，但是LOV 1或LOV 2都可以介导phot 2的磷酸化。我们还知道LOV 2足以恢复无效phot 1突变体的向光性反应，但不知道其他任何反应。该项目的第一个主要目标是理清两种向光蛋白和各个LOV结构域在每个响应中的作用。第二个主要目标是确定phot 2的细胞和亚细胞分布（我们已经有了phot 1的信息），这些信息是了解其功能的基础。最后，我们希望在一个或多个反应中识别下游反应伙伴，并确定不同反应的信号通路的分歧。向光蛋白对正常植物生长和发育至关重要，原因如下：首先，向光性是一种确保最大限度地收获光合作用光能的机制，显然对于最大限度地提高植物生长无论出于何种目的--食物、结构材料、纤维、药物生产等--都很重要。其次，对于像棉花这样的植物，其叶子跟踪太阳并保持与入射阳光的方向成直角，太阳的角度改变，成熟叶子的太阳能跟踪具有相同的功能：使光合作用最大化从而促进植物生长。第三，叶片展开是光合作用的绝对必要条件。在缺乏光促素的情况下，叶子矮小和卷曲，功能严重受损。第四，调节叶细胞中叶绿体位置的能力是必不可少的：a）通过在昏暗的光下最小化相互遮蔽来最大化光合作用，和B）通过在非常明亮的光下最大化相互遮蔽来防止叶绿体的光损伤。最后，气孔开放的光调节控制了光合作用吸收二氧化碳的速率，以平衡水分的损失。如果没有这种调节，正常的光合作用功能将严重受损。因此，沿着其他已知的植物光感受器，光敏色素和隐花色素，向光蛋白在植物发育和体内平衡中起着至关重要的作用。它们在人类福祉方面的作用显然是重要的。

The phototropins (phot1 and phot2) are a unique family of plant photoreceptors that bind two molecules of flavin mononucleotide (FMN) as their chromophores. Blue and UV-A light cause rapid autophosphorylation of these photoreceptors. In all plans thus far investigated, phot1 is associated with the plasma membrane and preliminary evidence indicates that phot2 is as well. The phototropins participate in phototropism (growth toward a blue or UV-A light source), light perception for chloroplast movement (avoidance by mutual shading in high light, moving to maximize light interception in low light), stomatal opening, rapid inhibition of growth of dark-grown seedlings, and likely solar tracking in those species that have that response. The FMN chromophores are bound tightly in specialized domains designated LOV1 and LOV2 (domains found in signaling proteins that respond to Light, Oxygen, or Voltage). The phototropin LOV domains undergo a unique photochemistry involving light-activated formation of a flavin triplet state that decays within microseconds to a metastable signaling state, a cysteinyl adduct at the C94a) carbon of the FMN. This signaling state decays to the dark state in a few seconds or minutes depending on which LOV domain is involved. The project will explore how this identical initial photochemistry (adduct formation) and biochemistry (autophosphorylation) can activate such different processes, some requiring cell-cell communication and other entirely cell autonomous. It will also explore the roles of the individual LOV domains in these various processes. We already know that only LOV2 is required for full autophosphorylation in phot1, but that either LOV1 or LOV2 can mediate phosphorylation of phot2. We also know that LOV2 is sufficient to restore the phototropic response in a null phot1 mutant but do not know for any of the other responses. Sorting out the roles of the two phototropins and of the individual LOV domains in each of the responses is the first major goal of the project. A second major goal is to determine the cellular and subcellular distribution of phot2 (we already have this information for phot1), information basic to an understanding of its function. Finally we hope to identify downstream reacting partners in one or more of the responses and determine where the signaling pathways diverge for the different responses. The phototropins are vital to normal plant growth and development for several reasons: first, phototropism is a mechanism to insure maximal harvest of light energy for photosynthesis, clearly of importance in maximizing plant growth for whatever purpose-food, structural materials, fiber, production of medicinal drugs, etc. Second, for plants like cotton that have leaves that track the sun and remain at right angles to the direction of the incident sunlight the sun's angle changes, solar tracking of mature leaves serves the same function: maximizing photosynthesis and hence plant growth. Third, leaf expansion is an absolute requisite for photosynthesis. In the absence of the phototropins, leaves are dwarfed and curled, and severely impaired in their function. Fourth, the capacity to regulate the position of the chloroplasts in the leaf cells is essential a) to maximize photosynthesis by minimizing mutual shading in dim light, and b) to prevent photodamage to the chloroplasts by maximizing mutual shading in very bright light. Finally, light regulation of stomatal opening controls the rate at which carbon dioxide is taken in for photosynthesis balanced against loss of water. Without this regulation, normal photosynthetic function would be severely impaired. Thus along with the other known plant photoreceptors, the phytochromes and cryptochromes, the phototropins play a vital role in plant development and homeostasis. Their role in human welfare is clearly a major one.