PDF(Pubmed)
Abstract：
Artificial or synthetic organelles are a key challenge for bottom-up synthetic biology. So far, synthetic organelles have typically been based on spherical membrane compartments, used to spatially confine selected chemical reactions. In vivo, these compartments are often far from being spherical and can exhibit rather complex architectures. A particularly fascinating example is provided by the endoplasmic reticulum (ER), which extends throughout the whole cell by forming a continuous network of membrane nanotubes connected by three-way junctions. The nanotubes have a typical diameter of between 50 and 100 nm. In spite of much experimental progress, several fundamental aspects of the ER morphology remain elusive. A long-standing puzzle is the straight appearance of the tubules in the light microscope, which form irregular polygons with contact angles close to 120°. Another puzzling aspect is the nanoscopic shapes of the tubules and junctions, for which very different images have been obtained by electron microcopy and structured illumination microscopy. Furthermore, both the formation and maintenance of the reticular networks require GTP and GTP-hydrolyzing membrane proteins. In fact, the networks are destroyed by the fragmentation of nanotubes when the supply of GTP is interrupted. Here, it is argued that all of these puzzling observations are intimately related to each other and to the dimerization of two membrane proteins anchored to the same membrane. So far, the functional significance of this dimerization process remained elusive and, thus, seemed to waste a lot of GTP. However, this process can generate an effective membrane tension that stabilizes the irregular polygonal geometry of the reticular networks and prevents the fragmentation of their tubules, thereby maintaining the integrity of the ER. By incorporating the GTP-hydrolyzing membrane proteins into giant unilamellar vesicles, the effective membrane tension will become accessible to systematic experimental studies.
摘要：
人工或合成细胞器是自下而上合成生物学的关键挑战。到目前为止,合成细胞器通常基于球形膜隔室，用于在空间上限制选定的化学反应。在体内,这些隔室通常远非球形，并且可以表现出相当复杂的结构。一个特别令人着迷的例子是内质网（ER），它通过形成由三通连接的膜纳米管的连续网络而延伸到整个细胞。纳米管具有在50和100nm之间的典型直径。尽管实验取得了很大进展，ER形态的几个基本方面仍然难以捉摸。一个长期存在的难题是在光学显微镜下直管的外观，形成接触角接近120°的不规则多边形。另一个令人费解的方面是细管和接头的纳米级形状，通过电子显微镜和结构化照明显微镜获得了非常不同的图像。此外，网状网络的形成和维持都需要GTP和GTP水解膜蛋白。事实上,当GTP的供应中断时，纳米管的碎片会破坏网络。这里,有人认为，所有这些令人费解的观察结果彼此密切相关，并且与锚定在同一膜上的两个膜蛋白的二聚化密切相关。到目前为止,这种二聚化过程的功能意义仍然难以捉摸，因此，似乎浪费了很多GTP。然而,这个过程可以产生有效的膜张力，稳定网状网络的不规则多边形几何形状，并防止它们的小管破碎，从而保持ER的完整性。通过将GTP水解膜蛋白整合到巨大的单层囊泡中，有效的膜张力将成为系统的实验研究。
