Electron Cryo-Tomography

CryoEM of bacterial secretion systems: A primer for microbiologists

"CryoEM" has come of age, enabling considerable structural insights into many facets of molecular biology. Here, we present a primer for microbiologists to understand the capabilities and limitations of two complementary cryoEM techniques for studying bacterial secretion systems. The first, single particle analysis, determines the structures of purified protein complexes to resolutions sufficient for molecular modeling, while the second, electron cryotomography and subtomogram averaging, tends to determine more modest resolution structures of protein complexes in intact cells. We illustrate these abilities with examples of insights provided into how secretion systems work by cryoEM, with a focus on type III secretion systems.

Insights into the evolution of bacterial flagellar motors from high-throughput in situ electron cryotomography and subtomogram averaging

In situ structural information on molecular machines can be invaluable in understanding their assembly, mechanism and evolution. Here, the use of electron cryotomography (ECT) to obtain significant insights into how an archetypal molecular machine, the bacterial flagellar motor, functions and how it has evolved is described. Over the last decade, studies using a high-throughput, medium-resolution ECT approach combined with genetics, phylogenetic reconstruction and phenotypic analysis have revealed surprising structural diversity in flagellar motors. Variations in the size and the number of torque-generating proteins in the motor visualized for the first time using ECT has shown that these variations have enabled bacteria to adapt their swimming torque to the environment. Much of the structural diversity can be explained in terms of scaffold structures that facilitate the incorporation of additional motor proteins, and more recent studies have begun to infer evolutionary pathways to higher torque-producing motors. This review seeks to highlight how the emerging power of ECT has enabled the inference of ancestral states from various bacterial species towards understanding how, and `why', flagellar motors have evolved from an ancestral motor to a diversity of variants with adapted or modified functions.