Co-evolution and molecular adaptation of the MLO-EXO70 functional module in the course of plant terrestrialization

MLO蛋白是作为钙通道运作的完整膜蛋白，是光合真核生物所特有的。在陆地植物中，它们通常在每个物种中以多序列多样化的同种异构体存在，并有助于基本的生理过程，如根的同种形态形成和向地性、施肥以及病原体防御。胞囊是一种进化上保守的多亚基蛋白复合物，存在于所有真核生物中。与其他分类群不同，在陆地植物中，EXO70胞囊亚基存在于多个序列多样化的亚型中。我们最近发现拟南芥MLOs和EXO70s形成了一个功能模块，涉及这些蛋白质的物理关联，并可能控制局灶分泌，例如细胞壁相关货物。我们假设MLO-EXO70功能模块的关键特征是在植物陆地化过程中进化而来的。因此，我们建议分析Chara braunii （Charophyceae）和Mesotaenium endlicherianum （Zygnematophyceae）的MLO-EXO70模块的共同进化和分子适应，这些模块作为所谓的ZCC级/进化枝的现存代表，标志着生物体从水到陆地的过渡。C. braunii和M. endlicherium基因组编码的mlo样蛋白（分别为7个和3个）在大小和膜拓扑结构方面与典型陆地植物的mlo样蛋白部分显着不同。此外，这些基因组分别编码两个原型EXO70亚基。利用这组来自陆地植物姐妹种的蛋白质，我们将评估最近发现的MLOs钙通道活性，它们与细胞质钙传感器钙调蛋白相关联的能力，以及它们与特定EXO70家族成员的同工型优先相互作用。我们打算进一步探索MLO和EXO70基因在不同条件下在复杂的C. braunii菌体各部分的表达模式，并研究这些基因在C. braunii物种中，更广泛地说，在Charophyceae中，这些基因的自然遗传变异。总之，这些研究将揭示在生命从水到陆地的转变过程中，MLO-EXO70功能模块的共同进化和分子适应的全球图景。

MLO proteins are integral membrane proteins that operate as calcium channels and that are specific to photosynthetic eukaryotes. In land plants, they are typically present in multiple sequence-diversified isoforms per species and contribute to essential physiological processes such as root thigmomorphogenesis and gravitropism, fertilization, as well as pathogen defense. Exocyst is an evolutionarily conserved multisubunit protein complex found in all eukaryotes. Different from other taxa, in land plants the EXO70 exocyst subunit is present in multiple sequence-diversified isoforms. We recently discovered that Arabidopsis thaliana MLOs and EXO70s form a functional module that involves physical association of these proteins and that likely governs focal secretion, e.g. of cell wall-related cargo. We hypothesize that key features of the MLO-EXO70 functional module evolved in the course of plant terrestrialization. We, therefore, propose analyzing the co-evolution and molecular adaptation of the MLO-EXO70 module in Chara braunii (Charophyceae) and Mesotaenium endlicherianum (Zygnematophyceae), which as an extant representatives of the so-called ZCC grade/clade mark the organismal transition from water to land. The C. braunii and M. endlicherianum genomes code for MLO-like proteins (seven and three, respectively) that in part markedly differ from their canonical land plant counterparts regarding size and membrane topology. Further, these genomes encode two prototypical EXO70 subunits each. Using this set of proteins derived from sister species of land plants, we will assess the recently discovered calcium channel activity of MLOs, their well-established ability to associate with the cytoplasmic calcium sensor calmodulin, and their isoform-preferential interaction with specific EXO70 family members. We further intend to explore the expression patterns of MLO and EXO70 genes in the various parts of the complex C. braunii thallus in different conditions and to examine the natural genetic variation of these genes within the species C. braunii and, more broadly, within the Charophyceae. Together, these studies will reveal a global picture regarding the presumed co-evolution and molecular adaptation of the MLO-EXO70 functional module in the course of transition of life from water to land.