步道内运输(IFT)是一种高度保守的机制,用于纤毛内货物的马达驱动运输,但是这种货物是如何选择性地运输到纤毛的,目前尚不清楚。WDR35/IFT121是IFT-A复合物的组成部分,以其在睫状逆行运输中的作用而闻名。在没有WDR35的情况下,会形成小的突变纤毛,但无法富集各种类型的纤毛膜蛋白。在Wdr35小鼠突变体中,非核心IFT-A组分降解,并且核心组分在纤毛基底处积累。我们揭示了WDR35和其他IFT-A亚基与α和β'COPI涂层亚基的深层序列同源性,并证明了无法与Wdr35突变纤毛融合的“无涂层”囊泡的积累。我们确定重组非核心IFT-As可以直接与脂质结合,并为WDR35提供新的外壳功能的第一个原位证据,可能与其他IFT-A蛋白一起。输送纤毛伸长所需的纤毛膜货物。
大多数人类细胞在其表面上至少有一个小的毛发状结构,称为纤毛。这些结构可以充当天线,并允许细胞感测来自身体其余部分的信号。要做到这一点,它们含有与细胞其他部分不同的蛋白质。纤毛的含量取决于这些蛋白质在纤毛内外的调节传递过程,称为步内运输或IFT,其中涉及由几种蛋白质组成的大型复合物。这种复合物使货物蛋白在纤毛的基部和尖端之间来回穿梭。然而,纤毛蛋白不在纤毛中产生;相反,它们在细胞的不同部分制成,然后被运输到纤毛基部。在他们进入纤毛的时候,他们被认为绑定到组装的IFT“火车”,并通过睫状门运输到纤毛需要的位置。IFT机制的组成部分之一是一种称为WDR35的蛋白质,也称为IFT121。如果编码这种蛋白质的基因有缺陷或缺失,它导致人类和小鼠的严重疾病,包括一系列潜在致命的骨骼发育不良。有趣的是,没有WDR35,细胞不能建立功能性纤毛。这种蛋白质的缺乏不仅会破坏IFT,阻止某些纤毛蛋白及其相关膜进入纤毛;它还会导致“交通堵塞”,从细胞中的运输中间体堆积到纤毛。目前还不清楚为什么IFT的一个组成部分的突变会产生这种效果,提出了一个问题,即WDR35还是整个IFT,在将货物蛋白带入纤毛中具有另一个作用。为了理解这种现象,Quidwai等人。分析了WDR35和其他IFT蛋白的结构,发现它们与称为COPI的蛋白质复合物非常相似,参与在细胞周围运输膜蛋白。当某些蛋白质是新制造的,它们被储存在被称为囊泡的小脂质气泡中,然后选择性地移动到需要蛋白质的地方。COPI包裹这些囊泡,在一个叫做囊泡运输的过程中帮助他们到达他们需要去的地方。Quidwai等人。发现WDR35和其他IFT蛋白能够与特定类型的脂质分子结合,这表明它们也可能有助于一种形式的囊泡运输。的确,当在实验室中生长的小鼠细胞经过基因工程改造,因此它们不能产生WDR35时,无涂层囊泡在纤毛的基部周围积累。向这些突变细胞中添加WDR35挽救了囊泡向纤毛转运的这些缺陷,并允许形成功能性纤毛。这些结果提供了证据,表明WDR35可能与其他IFT蛋白,作为一个COPI样的复合物,将蛋白质输送到正在生长的纤毛。进一步的研究将研究这些将蛋白质运输到纤毛的囊泡的组成,并帮助查明它们的起源。Quidwai等人。这些发现不仅揭示了在纤毛功能障碍患者中发现的不同基因突变如何影响将蛋白质运输到纤毛内的不同步骤。它们还增加了我们对蜂窝路线图的理解,通过该路线图,细胞可以在周围穿梭积木,以组装这些重要的“天线”。
Intraflagellar transport (IFT) is a highly conserved mechanism for motor-driven transport of cargo within cilia, but how this cargo is selectively transported to cilia is unclear. WDR35/IFT121 is a component of the IFT-A complex best known for its role in ciliary retrograde transport. In the absence of WDR35, small mutant cilia form but fail to enrich in diverse classes of ciliary membrane proteins. In Wdr35 mouse mutants, the non-core IFT-A components are degraded and core components accumulate at the ciliary base. We reveal deep sequence homology of WDR35 and other IFT-A subunits to α and ß\' COPI coatomer subunits and demonstrate an accumulation of \'coat-less\' vesicles that fail to fuse with Wdr35 mutant cilia. We determine that recombinant non-core IFT-As can bind directly to lipids and provide the first in situ evidence of a novel coat function for WDR35, likely with other IFT-A proteins, in delivering ciliary membrane cargo necessary for cilia elongation.
Most human cells have at least one small hair-like structure on their surface called a cilium. These structures can act as antennae and allow the cell to sense signals from the rest of the body. To do this, they contain proteins that differ from the rest of the cell. The content of cilia depends on regulated delivery of these proteins in and out of cilia by a process called the intraflagellar transport or IFT, which involves a large complex made of several proteins. This complex shuttles the cargo proteins back and forth between the base and the tip of the cilia. However, ciliary proteins are not produced in the cilia; instead, they are made in a different part of the cell and then they are transported to the ciliary base. At the point where they enter the cilia, they were thought to bind to the assembling IFT ‘trains’ and be transported across the ciliary gate to the positions where they are needed in cilia. One of the components of the IFT machinery is a protein called WDR35, also known as IFT121. If the gene that codes for this protein is faulty or missing, it results in severe disorders in both humans and mice including a range of potentially lethal skeletal dysplasias. Interestingly, without WDR35, cells cannot build functional cilia. The absence of this protein not only disrupts IFT, stopping certain ciliary proteins and their associated membranes from entering cilia; it also causes a ‘traffic jam’ with a pile-up of transport intermediates from the place in cell where they are made to the cilia. It is unclear why a mutation in one of the components of the IFT would have this effect, raising the question of whether WDR35, or IFTs a whole, has another role in bringing the cargo proteins into the cilia. To understand this phenomenon, Quidwai et al. analysed the structure of WDR35 and other IFT proteins and found that they are very similar to a protein complex called COPI, which is involved in transporting membrane proteins around the cell. When certain proteins are newly made, they are stored in small lipid bubbles – called vesicles – that then selectively move to where the proteins are needed. COPI coats these vesicles, helping them get to where they need to go in a process called vesicular transport. Quidwai et al. found that WDR35 and other IFT proteins are able to bind to specific types of lipid molecules, suggesting that they might be assisting in a form of vesicle transport too. Indeed, when mouse cells grown in the lab were genetically engineered so they could not produce WDR35, coatless vesicles accumulated around the base of the cilia. Adding back WDR35 to these mutant cells rescued these defects in vesicle transport to cilia as well as allowed functional cilia to be formed. These results provide evidence that WDR35, likely with other IFT proteins, acts as a COPI-like complex to deliver proteins to growing cilia. Further research will investigate the composition of these vesicles that transport proteins to cilia, and help pinpoint where they originate. Quidwai et al.’s findings not only shed light on how different genetic mutations found in patients with cilia dysfunction affect different steps of transporting proteins to and within cilia. They also increase our understanding of the cellular roadmap by which cells shuttle building blocks around in order to assemble these important ‘antennae’.