Trachtenberg S, Gilad R
Department of Membrane and Ultrastructure Research, The Hebrew University of Jerusalem - Hadassah Medical School, PO Box 11272, Jerusalem 91120, Israel.
Mol Microbiol. 2001 Aug;41(4):827-48. doi: 10.1046/j.1365-2958.2001.02527.x.
The Mollicutes (Mycoplasma, Acholeplasma, and Spiroplasma) are the smallest, simplest and most primitive free-living and self-replicating known cells. These bacteria have evolved from Clostridia by regressive evolution and genome reduction to the range of 5.8 x 10(5)-2.2 x 10(6) basepairs (bp). Structurally, the Mollicutes completely lack cell walls and are enveloped by only a cholesterol containing cell membrane. The Mollicutes contain what can be defined as a bacterial cytoskeleton. The Spiroplasmas are unique in having a well-defined, dynamic, helical cell geometry and a flat, monolayered, membrane-bound cytoskeleton, which follows, intracellularly, the shortest helical line on the cellular coil. By applying cryo-electron-microscopy to whole cells, isolated cytoskeletons and cytoskeletal fibrils and subunits, as well as by selective extraction of cellular components, we determined, at a resolution of approximately 25 A, the cellular and molecular organization of the cytoskeleton. The cytoskeleton is assembled from a 59 kDa protein. The 59 kDa protein, has an equivalent sphere diameter of approximately 50 A. Given the approximately 100 A axial and lateral spacings in the cytoskeletal ribbons and the near-circular shape of the subunit, we suggest that the subunit is a tetramer of 59 kDa monomers; the tetramers assemble further into flat fibrils, seven of which form a flat, monolayered, well-ordered ribbon. The cytoskeleton may function as a linear motor by differential and coordinated length-changes of the fibrils driven by conformational changes of the tetrameric subunits, the shape of which changes from near circular to elliptical. The cytoskeleton controls both the dynamic helical shape and the consequent motility of the cell. A stable cluster of proteins co-purifies with the cytoskeleton. These apparent membrane and membrane-associated proteins may function as anchor proteins.
柔膜菌纲（支原体、无胆甾原体和螺原体）是已知最小、最简单且最原始的自由生活和自我复制的细胞。这些细菌是由梭菌通过退行性进化和基因组缩减演化而来，基因组大小缩减至5.8×10⁵ - 2.2×10⁶碱基对（bp）。在结构上，柔膜菌纲完全没有细胞壁，仅被一层含胆固醇的细胞膜包裹。柔膜菌纲含有可被定义为细菌细胞骨架的结构。螺原体的独特之处在于具有明确的、动态的螺旋形细胞形态以及一个扁平的、单层的、膜结合的细胞骨架，该细胞骨架在细胞内沿着细胞螺旋的最短螺旋线分布。通过对完整细胞、分离的细胞骨架以及细胞骨架纤维和亚基应用冷冻电子显微镜，并通过选择性提取细胞成分，我们以约25埃的分辨率确定了细胞骨架的细胞和分子组织。细胞骨架由一种59 kDa的蛋白质组装而成。这种59 kDa的蛋白质，其等效球直径约为50埃。考虑到细胞骨架带中约100埃的轴向和横向间距以及亚基近乎圆形的形状，我们认为该亚基是由59 kDa单体组成的四聚体；这些四聚体进一步组装成扁平纤维，其中七条形成一个扁平的、单层的、排列有序的带。细胞骨架可能通过由四聚体亚基构象变化驱动的纤维的差异和协调长度变化作为线性马达发挥作用，四聚体亚基的形状从近乎圆形变为椭圆形。细胞骨架控制着细胞的动态螺旋形状以及随之而来的运动性。一组稳定的蛋白质与细胞骨架共同纯化。这些明显的膜蛋白和膜相关蛋白可能作为锚定蛋白发挥作用。
