Integration of two RAB5 groups during endosomal transport in plants.
Integration of two RAB5 groups during endosomal transport in plants.
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作者：
Ito E;Ebine K;Choi SW;Ichinose S;Uemura T;Nakano A;Ueda T
RAB 5是真核细胞内体功能的关键调节因子。植物具有两个不同的RAB 5组，典型和植物独特的类型，其通过未知的抵消机制起作用。在这里，我们在拟南芥中鉴定了植物特有的RAB 5的效应分子ARA 6，我们将其命名为植物特有的RAB 5效应分子2（PUF 2）。内体上与经典RAB 5的优先共定位和遗传相互作用分析表明，PUF 2与经典RAB 5协调液泡运输，尽管PUF 2被鉴定为ARA 6的效应子。PUF 2与GTP结合的ARA 6和GDP结合的经典RAB 5的竞争性结合，以及与共享的激活因子VPS 9a的相互作用，表明ARA 6通过滴定PUF 2和VPS 9a来负调节经典RAB 5介导的液泡转运。这些结果表明RAB效应子具有独特且前所未有的功能，涉及两个RAB基团的整合以协调植物细胞中的内体运输。活细胞通常包含通过称为膜运输的过程将蛋白质、脂肪和其他生物分子传递给彼此的隔室。内体是膜运输的关键平台之一。这些结构从细胞外部积累分子，对它们进行分类，然后将它们重新引导回细胞表面或将它们发送到细胞内的其他隔室，在那里它们可以被分解。被称为RAB 5的蛋白质调节内体的许多活动。有些存在于广泛的生物体中，包括动物、真菌和植物，被称为“典型”RAB 5组。另一组RAB 5蛋白是陆地植物和一些绿色藻类所特有的。存在两个RAB 5组（即典型和植物独特）是植物细胞的显著特征。2011年，研究人员发现，植物特有的RAB 5可以干扰和抵消典型RAB 5的活动。然而，这些蛋白质如何做到这一点仍然不清楚。为了解决这个问题，Ito等人-包括2011年研究的几位研究人员-着手寻找与拟南芥中植物特有的RAB 5相互作用的蛋白质。这些实验确定了植物特有的RAB 5的一个伴侣，命名为PUF 2。出乎意料的是，进一步的实验表明，PUF 2也调节典型的RAB 5。PUF 2与RAB 5和激活RAB 5的蛋白质一起被发现在内体表面。值得注意的是，PUF 2还与活化因子和经典RAB 5的失活形式相互作用。基于这些发现，Ito等人提出，PUF 2作为一个里程碑，使失活的经典RAB 5接近其激活因子，这有助于激活经典RAB 5。他们认为，植物特有的RAB 5也竞争性地结合到界标上，并阻断了典型的RAB 5。膜运输是所有生物体的通用系统，但该系统似乎在不同生物体中定制。这些新发现进一步证明了陆生植物已经进化出一种独特的机制来调节其内体的活动。下一步是重建这个系统是如何进化的，并揭示它与植物特异性性状进化的相关性。
RAB5 is a key regulator of endosomal functions in eukaryotic cells. Plants possess two different RAB5 groups, canonical and plant-unique types, which act via unknown counteracting mechanisms. Here, we identified an effector molecule of the plant-unique RAB5 in Arabidopsis thaliana, ARA6, which we designated PLANT-UNIQUE RAB5 EFFECTOR 2 (PUF2). Preferential colocalization with canonical RAB5 on endosomes and genetic interaction analysis indicated that PUF2 coordinates vacuolar transport with canonical RAB5, although PUF2 was identified as an effector of ARA6. Competitive binding of PUF2 with GTP-bound ARA6 and GDP-bound canonical RAB5, together interacting with the shared activating factor VPS9a, showed that ARA6 negatively regulates canonical RAB5-mediated vacuolar transport by titrating PUF2 and VPS9a. These results suggest a unique and unprecedented function for a RAB effector involving the integration of two RAB groups to orchestrate endosomal trafficking in plant cells. Living cells often contain compartments that pass proteins, fats and other biological molecules to one another via a process called membrane trafficking. Endosomes are one of the key platforms of membrane trafficking. These structures accumulate molecules from the outside of the cell, sort them, and then redirect them back to the cell surface or send them to other compartments within the cell where they can be broken down. Proteins known as RAB5s regulate many of the activities of endosomes. Some are found in a wide range of organisms, including animals, fungi, and plants, and are referred to as the “canonical” RAB5 group. Another group of RAB5 proteins are unique to land plants and some green algae. The existence of two RAB5 groups (i.e. canonical and plant-unique) is a distinctive feature of plant cells. In 2011, researchers showed that a plant-unique RAB5 could interfere with and counteract the activities of a canonical RAB5. However, it remained ambiguous how these proteins could do this. To resolve this question, Ito et al. – who include several researchers from the 2011 study – set out to find proteins that interact with a plant-unique RAB5 from Arabidopsis thaliana. The experiments identified one partner of a plant-unique RAB5, which was named PUF2. Unexpectedly, further experiments revealed that PUF2 also regulates canonical RAB5. PUF2 was found on the surface of the endosome together with RAB5s and a protein that activates RAB5s. Notably, PUF2 also interacted with the activating factor and the inactive form of canonical RAB5. Based on these findings, Ito et al. propose that PUF2 acts as a landmark to bring inactive canonical RAB5 close to its activating factor, which helps to activate canonical RAB5. They suggest that the plant-unique RAB5 also competitively binds to the landmark and blocks the canonical RAB5. Membrane trafficking is a universal system for all living organisms, yet the system seems to be customized among different organisms. These new findings provide further evidence that land plants have evolved a unique mechanism to regulate the activities of their endosomes. The next step is to reconstruct how this system evolved and unravel its relevance to the evolution of plant-specific traits.