Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany.
Max Planck Institute for Medical Research, 69120 Heidelberg, Germany.
ACS Nano. 2023 Jul 11;17(13):11957-11968. doi: 10.1021/acsnano.3c01338. Epub 2023 Jun 28.
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.
人工或合成细胞器是从下到上的合成生物学的一个关键挑战。到目前为止，合成细胞器通常基于球形膜隔室，用于空间限定所选化学反应。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。内质网 (ER) 提供了一个特别引人入胜的例子，它通过形成由三向连接连接的膜纳米管连续网络，从而在整个细胞中延伸。纳米管的典型直径在 50 到 100nm 之间。尽管取得了许多实验进展，但 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小管在光显微镜下的笔直外观，它形成具有接近 120°的接触角的不规则多边形。另一个令人困惑的方面是小管和连接的纳米级形状，通过电子显微镜和结构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 供应中断时，网络会被纳米管的断裂破坏。这里认为，所有这些令人困惑的观察结果都彼此密切相关，并且与锚定在同一膜上的两种膜蛋白的二聚化有关。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，并防止其纳米管的断裂，从而保持 ER 的完整性。通过将 GTP 水解膜蛋白纳入巨大的单层囊泡中，可以对有效膜张力进行系统的实验研究。
