想想来自四面八方的快递包裹，如何在物流中心汇集，尔后经分拣、精确投放到各地？我们凭借物流单上的详尽的地址标示保证了整个流程准确无误。在分子视觉下，其实细胞也是一个庞大的物流中心，可是在这里，没有人会“书写地址”，如何保证了“包裹邮件”能正确投放？

介绍：真核细胞含有多种与膜结合的细胞器，它们分别囊括有不同的内容物，行使了不同的功能。细胞器各自特性的维持依赖于其内容蛋白和脂的高度选择性，维持各自特性的中心则系于叫做囊泡的转运载体。需要弄清其中的机制：不但需要弄清楚特异的“货物”如何出芽进入囊泡而离开供体细胞器的，而且要明白囊泡如何投递给正确的受体细胞器。“货物”到达后，囊泡和其靶向的目的细胞器上的SNARE蛋白能驱动膜融合，因而在细胞器特异性选择中发挥作用。但是，是位于其上游的融合步骤，即囊泡“衔羁”于目的细胞器这一过程，使两边膜上的SNARE蛋白能相互作用、启动融合过程。就是这种为膜运输给予定向特异性的“衔羁”（tether）步骤的重要性，当前尚未弄清。Golgins蛋白是细胞器捕获特异类型囊泡的充分条件。我们假设：如果Golgins“衔羁”囊泡定向运往高尔基体，那我们将golgins蛋白重定位到线粒体上的话，将导致某些特定囊泡被异位捕获（于线粒体上）。免疫荧光证明（了这一假说）：线粒体上表达单一类型golgin蛋白可导致特殊“货物舱”出现异位捕获。电镜结果显示囊泡围绕着这些线粒体聚集分布。

基本原理：为了研究“衔羁”在膜运输特异性中的作用，我们聚焦在高尔基复合体上。高尔基复合体为多“腔室”的细胞器，是分泌和内吞转运通路的交汇口，其承接运往不同目的地的囊泡。高尔基体上有一大个保守的卷曲螺旋蛋白家族—golgins，golgins在高尔基体囊泡衔羁中可能发挥了重要作用。但是，一些golgin突变后呈现的轻微表型，使研究人员难于判定它们在“衔羁”过程中的作用。于是我们使用一种重定位策略来检测在囊泡“衔羁”过程中这类蛋白发挥了充分性作用，而不是必要性作用。选择了十种即便在非脊椎动物依旧保守的哺乳动物golgins，这些golgins分别位于高尔基体不同区域，通过把golgins蛋白的C末端高尔基定位结构域换成线粒体跨膜结构域，让它们在线粒体上异位表达。我们用于不同位置的运载“货物”囊泡的分布作为golgins蛋白在“衔羁”过程中作用的衡量指标(readout)。

结果：我们证明golgins蛋白的亚类(subsets)能将特定的内源性或外源性“货物”重新等位：从运往高尔基体变成运往线粒体。具体的讲，golgin-97, golgin-245和GCC88用于捕获由内体运往高尔基体的囊泡，GM130和GMAP210捕获内质网运往高尔基体的囊泡，golgin-84, TMF和GMAP210捕获高尔基体的常驻蛋白(resident proteins)。另外，电镜超微结构展现了装饰着特定golgins蛋白的囊泡膜围绕线粒体聚集的证据。这些数据表明golgins不但捕获囊泡，而且对来自内体，内质网和高尔基体等不同来源的囊泡具有捕获特异性。

结论：我们证实重新定位特定的golgins蛋白是导致特定类型转运囊泡异位运输的充分条件。因此多数golgins蛋白可决定特定的衔羁过程，它们在囊泡转运到达高尔基体的特异性中起了主要作用。另外，这种定位系统可作为分离这些存在时间短、靶向特异的转运囊泡的有用工具，从而为进一步研究膜转运的特异性提供了新途径。

你必须选择一两种高尔基“衔羁”

细胞内含有大量的各种类型的膜转运囊泡，每种囊泡都需要找到并与其正确的目的细胞器融合。不同囊泡如何与各自不同的靶向细胞器膜相融合喃，这类工作已有很多。在细胞内的膜结构转运通过高尔基复合体中，另外一层特异性就是需特定的膜“衔羁子”（tethers）。然而，这一“衔羁子”的重要性并未阐明。Wong和Munro使用了一种聪明的方法发现“衔羁子”保证了囊泡能正确定向。使用实验手段让“衔羁子”在线粒体表达，能劫持不同的转运囊泡，让它们不再运向正常目的地，而是运到线粒体上。

摘要：高尔基复合体由多个“腔室”组成，是在分泌和内吞的囊泡转运中处于中心地位的分拣站。运载着不同来源“货物”的转运囊泡能选择性接近不同的高尔基“腔室”，其中的机制尚未完全被弄清楚。我们发展了一中重定位和捕获方法，（用它）系统的研究了10种广泛存在的、保守的卷曲螺旋蛋白—golgin的囊泡衔羁活性。我们发现：在高尔基表面上定位不同的golgins亚类，其在捕获不同来源的囊泡上有选择性。这一发现证明：golgins作为体内的“衔羁”因子编码决定了衔羁的特异性，可能在高尔基复合体的膜转运中发挥了主要作用。

链接：http://www.sciencemag.org/content/346/6209/1256898.abstract

文题：The specificity of vesicle traffic to the Golgi is encoded in the golgin coiled-coil proteins

Introduction

The eukaryotic cell contains membrane-bound organelles with distinct functionality and composition. Preservation of organelle identity depends on the highly selective transfer of proteins and lipids between compartments. Central to this are transport carriers called vesicles. Mechanisms are required not only for the selective incorporation of specific cargos into vesicles as they bud off a donor organelle, but also for the correct delivery to an acceptor organelle. SNARE proteins on the vesicle and destination organelle drive membrane fusion after arrival and have been implicated in contributing to specificity in choice of organelle. However, upstream of the fusion step, a process called tethering is thought to initially attach the vesicle to the destination organelle and then bring it close to allow the SNARE proteins on opposite membranes to interact. The importance of tethering in conferring specificity to membrane traffic is currently unclear. Golgins are sufficient to capture specific classes of vesicle. We hypothesized that if the golgins tether vesicles destined for the Golgi, then their relocation to mitochondria should result in ectopic capture of specific classes of vesicle. Immunofluorescence demonstrates that the presence of a single golgin on mitochondria results in the ectopic capture of a specific cargo. Electron microscopy reveals that vesicle s accumulate around these mitochondria.

Rationale

To study the contribution of tethering to specificity in membrane trafficking, we focused on the Golgi apparatus. The Golgi complex is a multicompartment organelle at the intersection of secretory and endocytic trafficking pathways and so receives vesicles from a range of destinations. A family of well-conserved large coiled-coil proteins on the Golgi, the golgins, have been suggested to function as vesicle tethers at the Golgi. However, mild phenotypes of golgin mutants have presented a challenge for elucidating their in vivo roles. We thus used a relocation strategy to test for their sufficiency rather than necessity in vesicle tethering. Ten mammalian golgins that are conserved outside of vertebrates and found on different regions of the Golgi were ectopically expressed at the mitochondria through attachment to a mitochondrial transmembrane domain in place of their C-terminal Golgi targeting domain. We then used the distribution of cargo-laden vesicles originating from different locations as a readout for the golgins’ tethering activity.

Results

We demonstrate that subsets of golgins are capable of redirecting particular endogenous or exogenous cargo destined for the Golgi to an ectopic site, the mitochondria. Specifically, golgin-97, golgin-245, and GCC88 were able to capture endosome-to- Golgi cargos; GM130 and GMAP210 were able to capture endoplasmic reticulum (ER)–to-Golgi cargos; and golgin-84, TMF, and GMAP210 were able to capture Golgi resident proteins. Furthermore, electron microscopy yielded ultrastructural evidence for the accumulation of vesicular membranes around mitochondria decorated with specific golgins. These data suggest that not only do the golgins capture vesicles, but they also exhibit specificity toward vesicles of different origins-from the endosomes, from the ER, or from within the Golgi itself.

Conclusion

We have been able to demonstrate that relocation of specific golgins is sufficient to reroute specific classes of transport vesicles to an ectopic site. Thus, most golgins are sufficient to nucleate a specific tethering process, and hence they are likely to make a major contribution to the specificity of vesicle traffic arriving at the Golgi. In addition, this relocation system may be a useful tool for isolating specific transport vesicles that are normally short-lived, hence providing a route to further understanding of specificity in membrane traffic.

You've got to pick a Golgi tether or two

The inside of the cell contains a large variety of different membrane transport vesicles, each of which needs to find and fuse with its correct target destination. The detailed mechanism specifying which vesicle can fuse with which target membrane has been the subject of an enormous amount of research. An additional layer of specificity in intracellular membrane trafficking across the Golgi complex is thought to involve particular membrane “tethers.” However, the importance of these tethers has been unclear. Wong and Munro used a clever trick to reveal how specific tethers can indeed ensure correct vesicle destination. Tether proteins experimentally expressed on mitochondria hijacked different transport vesicles and diverted them from their normal destination to the mitochondria.

Abstract

The Golgi apparatus is a multicompartment central sorting station at the intersection of secretory and endocytic vesicular traffic. The mechanisms that permit cargo-loaded transport vesicles from different origins to selectively access different Golgi compartments are incompletely understood. We developed a rerouting and capture assay to investigate systematically the vesicle-tethering activities of 10 widely conserved golgin coiled-coil proteins. We find that subsets of golgins with distinct localizations on the Golgi surface have capture activities toward vesicles of different origins. These findings demonstrate that golgins act as tethers in vivo, and hence the specificity we find to be encoded in this tethering is likely to make a major contribution to the organization of membrane traffic at the Golgi apparatus.

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