Mass Mapping Of Macromolecular Assemblies

扫描透射电子显微镜（STEM）提供了一个通用的方法来确定分子质量，从而在大的蛋白质组装的亚基的安排。大分子被吸附到一个薄的支持膜和纳米尺寸的电子探针扫描整个试样，而收集的弹性散射信号。所得到的数字图像强度与样本的局部质量密度成比例。可以在低电子剂量下记录图像，而不会对感兴趣的结构造成显著的辐射损伤。我们已经使用这种方法来调查组成的螺原体柑橘，一个简单的细菌，这是感兴趣的，因为它的运动产生收缩的细胞骨架，这也作为一个线性电机的功能。这种马达可以作为理解其他细胞中细胞骨架功能的模型系统。此外，为了确定收缩线性马达的组织，我们现在正在表征膜结构并确定螺原体的蛋白质，脂质和核酸含量的组成。预计这些结果将提供有关维持细胞自由生活状态的最低要求的一些信息。

Scanning transmission electron microscopy (STEM) provides a versatile method for determining the molecular mass and hence the arrangement of subunits in large protein assemblies. Macromolecules are adsorbed onto a thin support film and a nanometer-sized electron probe is scanned across the specimen while the elastic-scattering signal is collected. The resulting digital image intensity is proportional to the local mass density of the specimen. Images can be recorded at low electron dose without significant radiation damage to the structures of interest. We have used this approach to investigate the composition of Spiroplasma citri, a simple bacterium, which is of interest because of its motility-producing contractile cytoskeleton, which also functions as a linear motor. This motor may serve as a model system for understanding the function of the cytoskeleton in other cells. In addition, to determining the organization of the contractile linear motor, we are now characterizing the membrane structure and determining Spiroplasma's composition in terms of protein, lipid and nucleic acid content. It is expected that the results will provide some information about the minimum requirements to sustain the cell's free-living state.