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Type III secretion systems: the bacterial flagellum and the injectisome
PMID: 26370933 Abstract
The flagellum and the injectisome are two of the most complex and fascinating bacterial nanomachines. At their core, they share a type III secretion system (T3SS), a transmembrane export complex that forms the extracellular appendages, the flagellar filament and the injectisome needle. Recent advances, combining structural biology, cryo-electron tomography, molecular genetics, in vivo imaging, bioinformatics and biophysics, have greatly increased our understanding of the T3SS, especially the structure of its transmembrane and cytosolic components, the transcriptional, post-transcriptional and functional regulation and the remarkable adaptivity of the system. This review aims to integrate these new findings into our current knowledge of the evolution, function, regulation and dynamics of the T3SS, and to highlight commonalities and differences between the two systems, as well as their potential applications.
Turns out, the eubacterial flagellum is homologous with simpler structures in the cell, such as the Type III secretory system. They are homologous structurally and genetically. There's an important hint in the fact that the eubacterial flagellum grows by movement of material from the bottom much as the secretory apparatus moves toxins and other materials through homologous structures. Both of these structures have simpler forms in the cell, showing transitional characteristics found in other evolutionary sequences.
The flagella of the Arachae are different, and are homologous with cell pilli, but are also transitional in form with other structures in Archae.