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

趋光素(Phot1和Pho2)是一个独特的植物光感受器家族，它结合黄素单核苷酸(FMN)的两个分子作为发色团。蓝光和UV-A光会引起这些光感受器的快速自动磷酸化。在到目前为止研究的所有计划中，Pho1与质膜有关，初步证据表明Pho2也是如此。向光素参与向光性(向蓝色或UV-A光源生长)，叶绿体运动的光感知(在高光下通过相互遮蔽来避免，在弱光下通过移动以最大限度地拦截光)，气孔开放，快速抑制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发色团紧密结合在被命名为LOV1和LOV2的专门结构域中(发现于对光、氧或电压做出反应的信号蛋白中的结构域)。趋光素LOV结构域经历独特的光化学过程，包括光激活形成黄素三重态，该状态在微秒内衰变到亚稳定信号状态，即FMN的C94a碳的半胱氨基加合物。该信令状态在几秒钟或几分钟内衰减到暗状态，具体取决于涉及哪个LOV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(自动磷酸化)如何激活这种不同的过程，其中一些需要细胞间的交流，另一些完全是细胞自主的。它还将探索单个LOV域在这些不同过程中的作用。我们已经知道，只有LOV2才能在PHY1中完全自磷酸化，但LOV1或LOV2都可以介导PHY2的磷酸化。我们还知道LOV2足以恢复空光突变体的光致反应，但不知道其他任何反应。该项目的第一个主要目标是找出两个趋光素和单个LOV结构域在每个反应中的作用。第二个主要目标是确定光子2的细胞和亚细胞分布(我们已经有了关于光子1的信息)，这是了解其功能的基本信息。最后，我们希望在一个或多个反应中确定下游反应伙伴，并确定不同反应的信号通路的哪里。向光素对植物的正常生长和发育至关重要，原因如下：第一，向光性是一种确保光合作用最大限度地获取光能的机制，显然在最大化植物生长方面非常重要，无论是出于什么目的--食物、结构材料、纤维、药物生产等。第二，对于棉花等植物，其叶片跟踪太阳，并与入射太阳光的方向保持直角变化，太阳跟踪成熟叶片具有相同的功能：最大化光合作用，从而使植物生长。第三，叶片展开是光合作用的绝对必要条件。在没有向光素的情况下，叶片变矮和卷曲，其功能严重受损。第四，调节叶绿体在叶细胞中位置的能力是至关重要的，a)通过最大限度地减少在暗光下的相互遮荫来最大化光合作用，以及b)通过在非常亮的光下最大限度地相互遮荫来防止对叶绿体的光损害。最后，光对气孔张开的调节控制着吸收二氧化碳进行光合作用的速率，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受到严重损害。因此，与其他已知的植物光感受器--光敏色素和隐色素一样，向光素在植物发育和动态平衡中发挥着至关重要的作用。它们在人类福利中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.