Role of Two Novel Pollen Tube Proteins in Polarized Rho GTPase Signalling

极化是一种允许细胞在结构和功能上形成不同极性的过程，它不仅对单细胞的正常行为和复制至关重要，而且对多细胞生物的发育也至关重要。活性Rho gtpase在细胞周围质膜特定区域的积累在极化过程中起着关键作用。Rho gtpase是一种可以通过控制与其直接相互作用的其他蛋白质（效应器）的活性来调节多种细胞过程的蛋白质。控制活性Rho gtpase在特定膜域局部积累的分子机制尚不清楚，这些蛋白如何调节细胞过程也仅在有限程度上得到澄清。烟草花粉管是一个非常有用的实验模型系统来研究极化。一种叫做Nt-Rac5的Rho GTPase特异性地聚集在这些高度细长的细胞一端的膜上，在那里它控制着发生在这个位置的细胞扩张。我们已经鉴定出两种新的烟草花粉管蛋白，其活性似乎依赖于Nt-Rac5的刺激，正如初步实验结果所表明的那样。这些实验表明，Nt-Rak1直接与活性Nt-Rac5结合，因此可能作为该蛋白的效应器。Nt-Rak1有能力通过将磷酸基团转移到其他蛋白质上来调节它们的活性，这一过程被称为磷酸化。有趣的是，我们发现Nt-Rak1不仅与Nt-PubPT（我们已经确定的一个假定的靶蛋白）相互作用，而且还使Nt-Rac5磷酸化。这表明Nt-Rak1作为Nt-Rac5的效应物，在调控Nt-PubPT功能的同时，调控Nt-Rac5的活性。蛋白质自身效应物对其活性的调节称为反馈调节，这一过程通常与极化有关。在此基础上，我们申请资助，进一步研究和建立Nt-Rak1和Nt-PubPT在烟草花粉管扩张端调控Nt-Rac5活性的功能，以及该蛋白对细胞过程的控制。我们相信这项工作将增强我们对极化的分子机制的理解，这是一个在单细胞和多细胞生物中具有基本功能的过程。

Polarization, a process that allows cells to organize structurally and functionally distinct poles, is essential not only for the normal behaviour and replication of single cells, but also for the development of multicellular organisms. The accumulation of active Rho GTPases at specific domains of the plasma membrane surrounding cells plays a key role in polarization. Rho GTPases are proteins that can regulate a variety of cellular process by controlling the activity of other proteins with which they directly interact (effectors). The molecular mechanisms that control the local accumulation of active Rho GTPases at specific membrane domains are not well understood, and how these proteins regulate cellular processes has only been clarified to a limited extent. Tobacco pollen tubes are an exceptionally useful experimental model system to investigate polarization. A Rho GTPase called Nt-Rac5 specifically accumulates at the membrane at one end of these highly elongated cells, where it controls cell expansion that occurs at this location. We have identified two novel tobacco pollen tube proteins, whose activities appear to depend on stimulation by Nt-Rac5, as suggested by the results of preliminary experiments. These experiments have shown that Nt-Rak1 directly binds to active Nt-Rac5, and is therefore likely to act as an effector of this protein. Nt-Rak1 has the ability to regulate the activity of other proteins by transferring phosphate groups to them, a process called phosphorylation. Interestingly, we have found that Nt-Rak1 not only interacts with Nt-PubPT, a putative target protein we have identified, but also phosphorylates Nt-Rac5. This suggests that Nt-Rak1 acts as a Nt-Rac5 effector, which regulates the function of Nt-PubPT, and at the same time modulates the activity of Nt-Rac5. The modulation of the activity of a protein by its own effector is called feedback regulation, a process that is often involved in polarization. With this proposal, we ask for funding to further investigate and establish the functions of Nt-Rak1 and Nt-PubPT in the regulation of the activity of Nt-Rac5 at the expanding end of tobacco pollen tubes, and in the control of cellular processes by this protein. We believe that this work will enhance out understanding of the molecular mechanisms underlying polarization, a process with essential functions in single cells and multicellular organisms.