A Unique Cytoplasmic ATPase Complex Defines the Type IV Secretion System Channel

Type IV secretion systems (T4SSs) are nanomachines that are widespread in bacteria. Despite their fundamental importance in processes such as DNA conjugation and the pathogenesis of plants, animals and humans, they are among the most complex and yet arguably the least understood secretion systems in the prokaryotic kingdom. Using live-fluorescence microscopy in conjunction with Cryo-Electron Tomography, we determined the in-situ structure of the Legionella pneumophila T4SS called Dot/Icm. Unexpectedly, we have discovered that the major ATPases energizing center in the cytosol of the bacterial cell creates a dynamic assembly and forms a unique central channel that is constructed by a hexameric array of dimeric proteins. We have showed that the ATPase DotB cycles between the cytosol and the Type IV machine, indicating that it is involved in energizing the Type IV apparatus once a signal is received to initiate protein translocation. Analysis of early-assembled intermediates showed an “outside-to-inside” assembly of the apparatus and revealed that DotB promotes conformational changes to the T4SS and the opening of the secretion channel. Our data elucidate how T4SSs function in their natural environment and provides new insights for future studies that are important for a complete understanding of host-pathogen interactions and antibiotic resistance processes.