Caught in the Act: Visualizing the Architecture of Bacterial Type IV Secretion System Machinery at the Cell-Cell Interface

T4SSs are extraordinarily versatile contact-dependent cargo delivery systems that are both phylogenetically and functionally diverse (2, 10, 11). These membrane-spanning systems are composed of conserved core complex subunits, as well as species-specific components that afford apparatus specialization and facilitate occupation of specific intracellular and extracellular niches (10-14). T4SSs can be divided into three subfamilies: (i) DNA conjugation machineries, (ii) DNA uptake/release systems that exchange DNA with the extracellular milieu, and (iii) effector translocation systems (2, 14, 15). Recent work has provided architectural and structural information highlighting the diversity that exists within core complexes of several T4SSs, including Escherichia coli conjugation machineries (14, 16), the Legionella pneumophila Dot/Icm T4SS (17), and the H. pylori cag T4SS (18). However, these investigations relied on the use of negative stain electron microscopy (EM) to image purified T4SS protein complexes produced in the absence of host cell contact, and attempts to resolve the structure of T4SS-assocaited pilus appendages have been unsuccessful.