Bacterial secretion systems all tackle the challenge of secreting substrates from the cytoplasm out of the cell. Many unrelated secretion systems have evolved, strikingly diverse in their structures and mechanisms, from the small pump-like type II secretion systems to the colossal harpoon-like type VI secretion systems. What makes secretion systems intriguing also makes them difficult to study, however: they are cell envelope-associated, frequently contain moving or transiently associated components, and are very large.Cryogenic electron microscopy (CryoEM) is ideally poised to study secretion systems. CryoEM excels at determining the structures of large protein complexes, can discern different conformational states, and can image structures in situ. CryoEM has come of age in the past half-decade following a spectacular resolution revolution that changed cryoEM from low-resolution "blobology" to a methodology that can compete with X-ray crystallography and NMR for structure determination of biological molecules (Kühlbrandt, 2014). Here, we provide a primer for microbiologists on these capabilities. We describe its basic principles, its two main forms, and what the two forms can and cannot achieve. We then highlight the contributions of cryoEM to understanding how the diverse microbial secretion systems work, focusing on type III secretion systems. We end with a forward-facing overview of future prospects. | Cr yoEMCryoEM is a family of techniques that determine the three-dimensional (3-D) structures of biological molecules from two-dimensional (2-D) electron micrographs. In basic terms, this is achieved by imaging the specimen from many different angles to facilitate piecing together
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