# Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure
## 5 Protein Synthesis, Export and Filament Elongation
### Flagellin Export and Assembly into the Growing Filament  
The export of flagellar proteins is a highly organized and well-controlled process conducted by the flagellar T3 SS apparatus located at the base of the flagellum, and powered by ATP and proton motive force (PMF) across the cytoplasmic membrane as the energy sources [146; 147; 148; 99; 136]. The T3 SS pumps unfolded flagellin monomers into the flagellum's central channel together with its cognate filament cap FliD, which are necessary for in vivo filament elongation. T3 SS also transports anti-sigma factor FlgM into the culture media, upon completion of the basal body assembly, allowing \(\sigma^{28}\) to transcribe flagellar filament genes [149; 150]. What constitutes the driving force for flagellin assembly in the growing filament remains an open question, considering that this process occurs far away from the cell where no conventional energy source is available. A possible answer was suggested by [PERSON] and coworkers, when they proposed a model of filament growth consisting of an inter-subunit chain mechanism in which the growth would be powered by the flagellin subunits themselves, since they would be linked head-to-tail, forming a chain that is pulled across the entire filament to the tip [151; 152]. According to this model, each newly linked unfolded flagellin subunit would be moved up from the gate into the filament pore by the entropic pulling force of the folded subunit anchored at its other end at the flagellum tip. In this way, successive rounds of subunit linking at the cell membrane export machinery would be coupled to the subunit folding at the tip to allow a continuous intrinsically energized subunit transit, resulting in a constant rate growth of the flagellum independent of the filament length [152; 153; 154]. However, this model has certain incompatibilities with the known biophysical properties of flagellum assembly [61; 155; 62]. In contrast to this model, [PERSON] et al. recently shed a light on the dynamic self-assembly process of filament growth, using in situ labelling and real-time immunostaining of elongating individual flagellar filaments [156]. They demonstrated that flagellar growth outside of the cell is controlled by a simple injection-diffusion mechanism. In this model, the flagellin monomers, which are at least partially \(\alpha\)-helical inside the channel, are pushed by a PMF-driven export apparatus into the flagellar channel and spread along the way passively by diffusion in one dimension [154; 157]. Hence, this new molecular mechanism proposed for bacterial flagella growth, provides a simple explanation to why the flagellar filament does not grow infinitely in the absence of any other length-control mechanism in place. They calculated that flagellar filament growth rate (initially \(\sim\)1700 amino acids per second) decays exponentially depending on filament length so, accordingly, the longer the flagellum gets, the slower it grows, as proposed initially by theoretical modeling and molecular dynamics simulations [155]. In addition, the inhibition of the PMF-dependent export apparatus revealed a major contribution of substrate injection in driving filament elongation. Although injection-diffusion is currently the most plausible model, further studies are needed to understand how the type III secretion system harnesses chemical energy to drive the movement of flagellin far outside the cell into the growing flagellum, and how the flagellin/channel friction interactions modulate the rate of flagellin monomers translocation. In this regard, recent analysis of the flagellin structure of _S. Typhimuriumium_ (PDB ID: 1 UCU) indicated that the hydrophilic inner surface of the narrow internal flagellar channel is lined with solvent-exposed side chains of charged and polar residues from the C-terminal D0 domain of FliC (residues Q84, N488, S491 and R494) [158].