Type IV secretion systems (T4SS) are widespread in bacteria and despite their fundamental importance in processes such as DNA conjugation and 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 T4SS of the respiratory pathogen Legionella pneumophila, 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 in that it 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. Our data changed the existing paradigm for how T4SS function and provides new insights for future studies that are important for a complete understanding of host pathogen interaction processes.
