Deciphering the molecular rules of evolutionary diversification of Intraflagellar Transport

纤毛（又名鞭毛）是从大多数真核细胞（包括人体的细胞类型）表面突出的普遍存在的细胞器。几乎所有纤毛都需要非膜结合的纤束内运输（IFT）来构建和发挥其功能。IFT是由所谓的IFT列车连续运输纤毛构件，由运动蛋白-2和动力蛋白-2发动机提供动力。IFT的一个特点是在纤毛基部和尖端的两个相反方向的马达具有位点特异性和互斥的激活和失活。越来越清楚的是，IFT序列的分层组装强制执行IFT过程的严格有序事件。在纤毛基地，从IFT- b综合体开始组装大型多兆道尔顿IFT列车。这个IFT-B复合物构成了支撑IFT-A复合物和动力蛋白-2马达的骨架。在装配的最后一步，利用驱动蛋白-2电机将列车移动到纤毛尖端。虽然IFT- b和IFT- a复合物与体内的动力蛋白-2和动力蛋白-2活性有关，但它们在分子水平上对IFT马达的调节的具体贡献仍在很大程度上未知。我们还清楚地发现，各自的IFT-B和IFT-A复合物的组成具有物种特异性和高度多样性。后者引发了各种生物体如何部署各自的IFT亚基来协调体内IFT过程的有序步骤的问题。在这里，我们研究并比较了嗜热T.和假单胞T.的IFT。尽管是同一个进化超级群体的成员，这些生物部署了一组惊人不同的IFT亚基和运动蛋白-2马达，但最终构建了本质上相同的细胞超微结构，即运动纤毛。T. pseudonana模型特别有趣，因为它消除了大多数IFT亚基，但保留了仅使用IFT- b复合体亚基子集来协调IFT过程的能力。这个“最小的”IFT模型可能会告诉我们功能灵活性的极限，这些灵活性可以被编码到一组特定的蛋白质中，以创造复杂的生物功能。从我们的初步数据中可以明显看出，来自“最小”假单胞菌和“典型”嗜热单胞菌模型的激酶2同源物显示出截然不同的自我调节和动力学特性。然而，所有的kinesin-2发动机都被期望遵守IFT高度有序步骤的规则。描述这些关键步骤的分子要求将揭示将马达与特定的IFT亚基联系起来的规则，这些规则最终分别在“最小”伪伪t和“典型”嗜热t模型中产生IFT过程中事件的严格年表。这一建议将使我们更接近于推导出在给定生物体中建立有序的IFT过程的规则，该规则基于IFT亚基集和它所部署的激酶-2类型。

Cilia (aka flagella) are ubiquitous organelles that project from the surface of most eukaryotic cells, including the cell types in the human body. Virtually all cilia require the non-membrane bound IntraFlagellar Transport (IFT) for their construction and function. IFT is a continuous transport of ciliary building blocks by the so-called IFT trains that are powered by the kinesin-2 and dynein-2 motors. A hallmark of IFT is the site-specific and mutually exclusive activation and deactivation of the two oppositely directed motors at the ciliary base and tip. It is becoming increasingly clear that a hierarchical assembly of the IFT trains enforces the strictly ordered events of the IFT process. At the ciliary base, large multi mega-Dalton IFT trains are assembled starting with the IFT-B complex. This IFT-B complex constitutes the backbone that scaffolds the IFT-A complex and the dynein-2 motor. In the last step of the assembly, the kinesin-2 motor is recruited to move the trains towards the ciliary tip. Whereas IFT-B and IFT-A complexes have been linked to the kinesin-2 and dynein-2 activities in vivo, their specific contributions to the regulation of IFT motors remain largely unknown at the molecular level. It also became clear that the composition of the respective IFT-B and IFT-A complexes are species-specific and highly diverse. The latter provokes the question of how the various organisms deploy the respective IFT subunits to orchestrate the orderly steps of the IFT process in vivo. Here, we investigate and contrast IFT of the Chromalveolates T. thermophila and T. pseudonana. Despite being members of the same evolutionary super group, these organisms deploy an astonishingly different set of IFT subunits and kinesin-2 motors, yet end up building essentially the same cellular ultrastructure, a motile cilium. T. pseudonana model is of particular interest as it has eliminated most IFT subunits but retained the ability to orchestrate the IFT process using only a subset of subunits from the IFT-B complex. This ‘minimal’ IFT model will likely teach us the limits of functional flexibility that can be coded into a specific set of proteins to create complex biological function. As already evident from our preliminary data, the kinesin-2 orthologs from the ‘minimal’ T. pseudonana and ‘canonical’ T. thermophila models display vastly different self-regulatory and kinetic properties. Yet all kinesin-2 motors are expected to obey the rules of the highly orderly steps of IFT. Delineating the molecular requirements of these key steps will expose the rules of linking the motors to specific IFT subunits that ultimately give rise to the strict chronology of events during the IFT process in the ‘minimal’ T. pseudonana and ‘canonical’ T. thermophila models, respectively. This proposal will bring us closer to derive the rules of establishing an orderly IFT process in a given organism based on the set of IFT subunits and the type(s) of kinesin-2 that it deploys.