ICK (also known as CILK1) is a mitogen-activated protein kinase–like kinase localized at the ciliary tip. Its deficiency is known to result in the elongation of cilia and causes ciliopathies in humans. However, little is known about how ICK is transported to the ciliary tip. We here show that the C-terminal noncatalytic region of ICK interacts with the intraflagellar transport (IFT)–B complex of the IFT machinery and participates in its transport to the ciliary tip. Furthermore, total internal reflection fluorescence microscopy demonstrated that ICK undergoes bidirectional movement within cilia, similarly to IFT particles. Analysis of ICK knockout cells demonstrated that ICK deficiency severely impairs the retrograde trafficking of IFT particles and ciliary G protein–coupled receptors. In addition, we found that in ICK knockout cells, ciliary proteins are accumulated at the bulged ciliary tip, which appeared to be torn off and released into the environment as an extracellular vesicle. The exogenous expression of various ICK constructs in ICK knockout cells indicated that the IFT-dependent transport of ICK, as well as its kinase activity and phosphorylation at the canonical TDY motif, is essential for ICK function. Thus, we unequivocally show that ICK transported to the ciliary tip is required for retrograde ciliary protein trafficking and consequently for normal ciliary function.

中文翻译：

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纤毛内逆行蛋白运输需要通过鞭毛内运输机制顺行运输纤毛 MAP 激酶样 ICK/CILK1。


ICK（也称为 CILK1）是一种位于纤毛尖端的丝裂原激活蛋白激酶样激酶。已知它的缺乏会导致纤毛伸长并导致人类纤毛病。然而，人们对 ICK 如何转运至睫状体尖端知之甚少。我们在这里表明，ICK 的 C 端非催化区域与 IFT 机制的鞭毛内运输 (IFT)-B 复合物相互作用，并参与其向纤毛尖端的运输。此外，全内反射荧光显微镜表明 ICK 在纤毛内进行双向运动，类似于 IFT 颗粒。 ICK 敲除细胞的分析表明，ICK 缺陷严重损害 IFT 颗粒和纤毛 G 蛋白偶联受体的逆行运输。此外，我们发现在ICK敲除细胞中，纤毛蛋白在凸出的纤毛尖端处积累，这些蛋白似乎被撕裂并作为细胞外囊泡释放到环境中。 ICK 敲除细胞中各种 ICK 构建体的外源表达表明 ICK 的 IFT 依赖性转运及其激酶活性和典型 TDY 基序的磷酸化对于 ICK 功能至关重要。因此，我们明确表明，ICK 转运到睫状体尖端是逆行睫状体蛋白运输所必需的，因此也是正常睫状体功能所必需的。