Schmid Michael F, Paredes Angel M, Khant Htet A, Soyer Ferda, Aldrich Henry C, Chiu Wah, Shively Jessup M
National Center for Macromolecular Imaging, Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA.
J Mol Biol. 2006 Dec 1;364(3):526-35. doi: 10.1016/j.jmb.2006.09.024. Epub 2006 Sep 14.
Carboxysomes are polyhedral bodies consisting of a proteinaceous shell filled with ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO). They are found in the cytoplasm of all cyanobacteria and some chemoautotrophic bacteria. Previous studies of Halothiobacillus neapolitanus and Nitrobacter agilis carboxysomes suggest that the structures are either icosahedral or dodecahedral. To determine the protein shell structure more definitively, purified H. neapolitanus carboxysomes were re-examined by cryo-electron tomography and scanning transmission electron microscopy (STEM). Due to the limited tilt angles in the electron microscope, the tomographic reconstructions are distorted. Corrections were made in the 3D orientation searching and averaging of the computationally extracted carboxysomes to minimize the missing data effects. It was found that H. neapolitanus carboxysomes vary widely in size and mass as shown by cryo-electron tomography and STEM mass measurements, respectively. We have aligned and averaged carboxysomes in several size classes from the 3D tomographic reconstruction by methods that are not model-biased. The averages reveal icosahedral symmetry of the shell, but not of the density inside it, for all the size classes.
羧酶体是一种多面体结构，由充满1,5-二磷酸核酮糖羧化酶/加氧酶（RuBisCO）的蛋白质外壳组成。它们存在于所有蓝细菌和一些化能自养细菌的细胞质中。先前对那不勒斯嗜盐硫杆菌和敏捷硝化杆菌羧酶体的研究表明，其结构要么是二十面体，要么是十二面体。为了更确切地确定蛋白质外壳结构，通过冷冻电子断层扫描和扫描透射电子显微镜（STEM）对纯化的那不勒斯嗜盐硫杆菌羧酶体进行了重新检查。由于电子显微镜中的倾斜角度有限，断层扫描重建结果存在失真。在对通过计算提取的羧酶体进行三维方向搜索和平均时进行了校正，以尽量减少缺失数据的影响。冷冻电子断层扫描和STEM质量测量结果分别表明，那不勒斯嗜盐硫杆菌羧酶体在大小和质量上差异很大。我们通过无模型偏差的方法，对三维断层扫描重建中几个大小类别的羧酶体进行了对齐和平均。结果显示，所有大小类别的羧酶体外壳均具有二十面体对称性，但其内部密度不具有该对称性。
