structure of the Dot/Icm type IV secretion system by electron cryotomography.

Type IV secretion systems (T4SSs) are large macromolecular machines that translocate protein and DNA and are involved in the pathogenesis of multiple human diseases. Here, using electron cryotomography (ECT), we report the structure of the Dot/Icm type IVB secretion system (T4BSS) utilized by the human pathogen This is the first structure of a type IVB secretion system, and also the first structure of any T4SS While the Dot/Icm system shares almost no sequence similarity with type IVA secretion systems (T4ASSs), its overall structure is seen here to be remarkably similar to previously reported T4ASS structures (those encoded by the R388 plasmid in and the cag pathogenicity island in ). This structural similarity suggests shared aspects of mechanism. However, compared to the negative-stain reconstruction of the purified T4ASS from the R388 plasmid, the Dot/Icm system is approximately twice as long and wide and exhibits several additional large densities, reflecting type-specific elaborations and potentially better structural preservation .