Huecas Sonia, Ramírez-Aportela Erney, Vergoñós Albert, Núñez-Ramírez Rafael, Llorca Oscar, Díaz J Fernando, Juan-Rodríguez David, Oliva María A, Castellen Patricia, Andreu José M
Centro de Investigaciones Biológicas, CSIC, Madrid, Spain.
Instituto de Quimica Fisica Rocasolano, CSIC, Madrid, Spain.
Biophys J. 2017 Oct 17;113(8):1831-1844. doi: 10.1016/j.bpj.2017.08.046.
FtsZ is a self-assembling GTPase that forms, below the inner membrane, the mid-cell Z-ring guiding bacterial division. FtsZ monomers polymerize head to tail forming tubulin-like dynamic protofilaments, whose organization in the Z-ring is an unresolved problem. Rather than forming a well-defined structure, FtsZ protofilaments laterally associate in vitro into polymorphic condensates typically imaged on surfaces. We describe here nanoscale self-organizing properties of FtsZ assemblies in solution that underlie Z-ring assembly, employing time-resolved x-ray scattering and cryo-electron microscopy. We find that FtsZ forms bundles made of loosely bound filaments of variable length and curvature. Individual FtsZ protofilaments further bend upon nucleotide hydrolysis, highlighted by the observation of some large circular structures with 2.5-5° curvature angles between subunits, followed by disassembly end-products consisting of highly curved oligomers and 16-subunit -220 Å diameter mini-rings, here observed by cryo-electron microscopy. Neighbor FtsZ filaments in bundles are laterally spaced 70 Å, leaving a gap in between. In contrast, close contact between filament core structures (∼50 Å spacing) is observed in straight polymers of FtsZ constructs lacking the C-terminal tail, which is known to provide a flexible tether essential for FtsZ functions in cell division. Changing the length of the intrinsically disordered C-tail linker modifies the interfilament spacing. We propose that the linker prevents dynamic FtsZ protofilaments in bundles from sticking to one another, holding them apart at a distance similar to the lateral spacing observed by electron cryotomography in several bacteria and liposomes. According to this model, weak interactions between curved polar FtsZ protofilaments through their the C-tails may facilitate the coherent treadmilling dynamics of membrane-associated FtsZ bundles in reconstituted systems, as well as the recently discovered movement of FtsZ clusters around bacterial Z-rings that is powered by GTP hydrolysis and guides correct septal cell wall synthesis and cell division.
FtsZ是一种能自我组装的GTP酶，在内膜下方形成引导细菌分裂的细胞中部Z环。FtsZ单体头对尾聚合形成微管蛋白样动态原丝，其在Z环中的组织方式仍是一个未解决的问题。FtsZ原丝并非形成明确的结构，而是在体外横向缔合形成多态凝聚物，通常在表面成像。我们在此描述了溶液中FtsZ组装体的纳米级自组织特性，这些特性是Z环组装的基础，采用了时间分辨X射线散射和冷冻电子显微镜技术。我们发现FtsZ形成了由长度和曲率可变的松散结合细丝组成的束。单个FtsZ原丝在核苷酸水解时进一步弯曲，这通过观察到一些亚基之间曲率角为2.5 - 5°的大圆形结构得到突出体现，随后是由高度弯曲的寡聚体和直径为16亚基 - 220 Å的微环组成的解体终产物，这是我们通过冷冻电子显微镜观察到的。束中相邻的FtsZ细丝横向间距为70 Å，中间留有间隙。相比之下，在缺乏C末端尾巴的FtsZ构建体的直聚合物中，观察到细丝核心结构之间紧密接触（间距约为50 Å），已知C末端尾巴为FtsZ在细胞分裂中的功能提供了必不可少的柔性系链。改变内在无序的C尾连接子的长度会改变细丝间间距。我们提出，连接子可防止束中动态的FtsZ原丝相互粘连，使其保持类似于在几种细菌和脂质体中通过电子冷冻断层扫描观察到的横向间距的距离。根据该模型，弯曲的极性FtsZ原丝通过其C尾之间的弱相互作用，可能有助于在重构系统中膜相关FtsZ束的连贯踏车动力学，以及最近发现的由GTP水解驱动并引导正确隔膜细胞壁合成和细胞分裂的FtsZ簇围绕细菌Z环的移动。
