纤毛（或鞭毛）是以微管为结构骨架，由纤毛膜包被且突出于细胞表面的细胞器，普遍存在于真核生物中。纤毛功能众多，主要参与细胞运动及各类信号传导过程，因此纤毛结构或长度的异常会造成一系列的疾病。纤毛的组装与长度维持需要“鞭毛内运输”（IFT）系统的参与，对IFT过程及纤毛长度调控机制的研究，可以为纤毛相关疾病的预防及治疗提供理论依据。异源三聚体Kinesin-2（Kinesin-II）是正向IFT的马达蛋白，对于大多数纤毛的组装及维持至关重要。在不同生物中，正向IFT的速率差异很大（如：衣藻中正向IFT的速率为～2.2 μm/s，而哺乳动物细胞为～0.5 μm/s），然而目前尚不清楚这些速率差异背后的机制。并且，Kinesin-II速率对IFT过程以及纤毛长度的影响也不清楚。此外，Kinesin-II的结构特性是由两个不同的马达亚基组成，但目前对于Kinesin-II需要异源马达二聚体的生理学意义和分子基础仍不清楚。在本论文中，我们在体外和体内对人源Kinesin-II（HsKinesin-II）、衣藻源Kinesin-II（CrKinesin-II）和多种嵌合的Kinesin-II展开了研究。我们的研究表明，CrKinesin-II的运动亚基（FLA10和FLA8）可以分别形成同源二聚体，并且带有两个相同motor结构域的嵌合马达蛋白，能够在体外和体内持续运动。进一步探究发现，FLA8的同源二聚体无法与KAP结合，以及FLA10在衣藻细胞内无法独立于FLA8稳定存在，因此，这可能是CrKinesin-II需要两种马达亚基同时存在的原因。另外，通过对CrKinesin-II、HsKinesin-II以及嵌合Kinesin-II的体外和体内的运动分析，揭示了正向IFT速率的差异是由物种特异性的Kinesin-II造成的。此后，我们在衣藻细胞内构建带有人源motor结构域的嵌合Kinesin-II（KIF3B'/FLA10），使得正向IFT的速度降低近三倍，导致IFT注入速率发生类似比例的降低，但纤毛长度仅下降～15%。最后，通过建立数学理论模型，我们揭示了IFT速率与纤毛长度之间的非线性关系，并暗示纤毛中IFT蛋白的水平可以反过来调节IFT的注入速率，从而调控纤毛长度。综上所述，我们的研究揭示了在驱动IFT过程中需要CrKinesin-II形成异源二聚体马达亚基的分子机理，正向IFT在不同生物中速率不同的机制，以及马达蛋白运动速度与纤毛长度调控的关系。该研究对了解Kinesin-II的作用机制以及纤毛长度调控具有十分重要的意义。

Cilium (or flagellum) is a hair-like organelle based on microtubules that protrude from cell surface and perform a variety of biological functions, such as signal sensing, signal transduction and movement. The abnormal of ciliary structures or length would cause a series of diseases, which is known as ciliopathies. The assembly and length control of cilia require intraflagellar transport (IFT). Therefore, the research on the IFT process and the regulation mechanism of cilia length can provide a theoretical basis for the prevention and treatment of ciliopathies.Heterotrimeric Kinesin-2 (Kinesin-II) serves as the anterograde motor for IFT and is essential for ciliary assembly and maintenance. The rate of anterograde IFT varies significantly in different organisms. For example, it is ～2.2 μm/s in Chlamdyomonas while ～0.5 μm/s in mammalian cells. It is not clear what the difference between these velocities is determined. Furthermore, it is not clear whether and how the change in velocity would affect IFT and ciliary length. In addition, the structural characteristic of Kinesin-II is composed of non-identical motor subunits, but the underlying physiological and functional significance is still unclear. In the present study, we have studied human Kinesin-II (HsKinesin-II), Chlamydomonas Kinesin-II (CrKinesin-II) and various chimeric Kinesin-IIs in vitro and/or in vivo. We demonstrated that the motor subunits of CrKinesin-II (FLA10 and FLA8) could self-interact respectively and the chimeric motors with two identical motor domains could function in vitro and in vivo. Further research revealed that the requirement for heterodimer formation of CrKinesin-II for IFT was likely attributed to the homodimer could not interact with KAP (for FLA8) or stable (for FLA10)，which highlighted the necessity of heterodimeric motor in Kinesin-II for IFT. In addition, by comparing the motility of CrKinesin-II, HsKinesin-II and various chimeric Kinesin-IIs, we found that the difference in anterograde IFT velocity was primarily determined by the species-specific of Kinesin-II. We have created chimeric Kinesin-II motors with the motor domains of CrKinesin-II replaced by their counterparts of HsKinesin-II, which reduced velocities of CrKinesin-II and anterograde IFT both by nearly 3-fold. The reduced velocity of anterograde IFT resulted in a similar reduction in IFT injection rate, but only a slight reduction in ciliary length (～15%). Finally, we established a theoretical model with the assumption of limited IFT proteins (e.g. motors) and diffusion delays the return of Kinesin-II, which would deplete Kinesin-II available for anterograde transport, revealing the non-linear relationship between the speed of IFT and the length of cilia and implying the level of IFT protein in cilia could in turn regulate IFT injection rate.In summary, our studies revealed the mechanism for requiring heterodimeric motor of Kinesin-II in driving IFT, the mechanism underlying different anterograde IFT velocities among organisms, and the mechanisms for motor speed and ciliary length control.