Assembly of a polar domain - the role of electrostatic interactions at the plasma membrane during the initiation of tip growth in Arabidopsis roots

细胞极性是几乎所有活细胞的统一特征。它的特点是细胞功能的局部专门化，其中包括为生长或运动提供方向性。在质膜上，对称性破坏，从而发生功能极化，发生微尺度和中尺度的区域，可以是持久的，也可以是短暂的。如何实现地方膜的专业化是几十年来一直争论的问题。由于大多数细胞几乎不是未极化的，所以研究极性的起始和极域的形成是具有挑战性的。植物根的特征是一种细胞类型，称为毛细胞或毛母细胞，它从伸长过渡到顶端生长，因此需要在质膜上建立一个新的极轴和极域。这个极区的蛋白质组装是如何协调的，以及哪些信号或受体触发了毛发形成的启动，目前还不清楚。我们利用这一过程的高度可预测性来研究特定蛋白质和脂类的招募到新兴的根毛起始域。我们的第一个目标是分析蛋白质的逐步组装，并确定沿着头发发育时间轴的特定蛋白质-蛋白质相互作用。我们的第二个目标是通过研究簇状磷脂酰肌醇4，5-二磷酸(PIP2)的作用来揭示膜蛋白募集的机制。PIP2是一类位于质膜上的负电荷磷脂。PIP2簇可以通过静电相互作用驱动蛋白质募集，细胞可以通过改变膜电位或胞内离子浓度来快速调节蛋白质募集。我们的第三个目标是揭示一种新的受体样激酶在启动毛发生长中的功能。其特定的基因表达，其在根毛起始区的早期出现，以及在功能获得突变体中毛发形成的增加，表明该RLK参与了触发或协调新根毛的形成。综上所述，通过解剖极性毛发起始域组装过程中的动态相互作用网络，通过研究蛋白质动态募集的机制和这一过程的潜在协调者，我们旨在为在质膜上建立中尺度结构域和启动极性生长提供新的见解。

Cell polarity is a unifying feature of virtually all living cells. It is characterized by localized specialization of cellular functions that provide, among others, directionality for growth or motility. At the plasma membrane, symmetry breaking and, hence, functional polarization, occurs microscale- and mesoscale domains that can be persistent or short-lived. How local membrane specialization is achieved is under debate for decades. Since most cells are hardly ever unpolarized it is challenging to investigate the initiation of polarity and the formation of polar domains.Plant roots feature a cell type, called hair cells or trichoblasts, that transition from elongation to tip growth and, thus, need to establish a new polar axis and a polar domain at the plasma membrane. How the protein assembly at this polar domain is coordinated and which signals or receptors trigger the initiation of hair formation is not well understood. We take advantage of the high predictability of this process to study the recruitment of specific proteins and lipids to the emerging root hair initiation domain. Our first goal is to analyze the step-wise assembly of proteins and determine specific protein-protein interactions along the time axis of hair development. Our second goal is to unveil mechanisms of the membrane protein recruitment by investigating the role of clustered phosphatidylinositide 4,5-bisphosphate (PIP2), a class of negatively charged phospholipids at the plasma membrane. PIP2 clustering can drive protein recruitment via electrostatic interactions, which can be modulated rapidly by the cell through changes in membrane potential or cytosolic ion concentration.Our third goal is to unveil the function of a novel receptor-like kinase in the initiation of hair emergence. Its specific gene-expression, its early appearance at the root hair initiation domain and an increased hair formation in gain-of-function mutants point toward an involvement of this RLK in triggering or coordinating the formation a new root hair. Taken together, by dissecting the dynamic interaction networks during the assembly of the polar hair initiation domain, by studying a mechanism for dynamic protein recruitment and by investigating a potential coordinator of this process we aim to provide new insights in the establishment of mesoscale domains at the plasma membrane and the initiation of polar growth.