Department of Microbiology and Molecular Genetics, IMRIC, The Hebrew University–Hadassah Medical School, PO Box 12272, Jerusalem 91120, Israel.
J Mol Biol. 2011 Jul 8;410(2):194-213. doi: 10.1016/j.jmb.2011.04.067. Epub 2011 May 13.
Spiroplasmas belong to the class Mollicutes, representing the minimal, free-living, and self-replicating forms of life. Spiroplasmas are helical wall-less bacteria and the only ones known to swim by means of a linear motor (rather than the near-universal rotary bacterial motor). The linear motor follows the shortest path along the cell's helical membranal tube. The motor is composed of a flat monolayered ribbon of seven parallel fibrils and is believed to function in controlling cell helicity and motility through dynamic, coordinated, differential length changes in the fibrils. The latter cause local perturbations of helical symmetry, which are essential for net directional displacement in environments with a low Reynolds number. The underlying fibrils' core building block is a circular tetramer of the 59-kDa protein Fib. The fibrils' differential length changes are believed to be driven by molecular switching of Fib, leading consequently to axial ratio and length changes in tetrameric rings. Using cryo electron microscopy, diffractometry, single-particle analysis of isolated ribbons, and sequence analyses of Fib, we determined the overall molecular organization of the Fib monomer, tetramer, fibril, and linear motor of Spiroplasma melliferum BC3 that underlies cell geometry and motility. Fib appears to be a bidomained molecule, of which the N-terminal half is apparently a globular phosphorylase. By a combination of reversible rotation and diagonal shift of Fib monomers, the tetramer adopts either a cross-like nonhanded conformation or a ring-like handed conformation. The sense of Fib rotation may determine the handedness of the linear motor and, eventually, of the cell. A further change in the axial ratio of the ring-like tetramers controls fibril lengths and the consequent helical geometry. Analysis of tetramer quadrants from adjacent fibrils clearly demonstrates local differential fibril lengths.
螺旋体属于柔膜体纲，代表着最小、自由生活和自我复制的生命形式。螺旋体是无壁的螺旋形细菌，也是唯一已知通过线性马达游动的细菌（而不是几乎普遍存在的旋转细菌马达）。线性马达沿着细胞的螺旋膜管的最短路径运动。该马达由一个由七个平行原纤维组成的平坦单层带状物组成，据信它通过原纤维的动态、协调和差分长度变化来控制细胞的螺旋性和运动性。这种变化导致螺旋对称性的局部扰动，这对于在低雷诺数环境中实现净定向位移是必不可少的。原纤维的核心构建块是由 59kDa 蛋白 Fib 组成的圆形四聚体。据信，原纤维的差分长度变化是由 Fib 的分子开关驱动的，从而导致四聚体环的纵横比和长度变化。通过低温电子显微镜、衍射、分离带状物的单颗粒分析以及 Fib 的序列分析，我们确定了构成细胞几何形状和运动基础的蜜蜂螺旋体 BC3 的 Fib 单体、四聚体、原纤维和线性马达的整体分子组织。Fib 似乎是一个具有两个结构域的分子，其 N 端的一半显然是一个球形的磷酸化酶。通过 Fib 单体的可逆旋转和对角线位移的组合，四聚体采用十字形非手性构象或环形手性构象。Fib 旋转的方向可能决定线性马达的手性，并最终决定细胞的手性。环型四聚体的纵横比的进一步变化控制着原纤维的长度和随之而来的螺旋几何形状。来自相邻原纤维的四聚体象限的分析清楚地表明了局部差分原纤维长度。
