Assembly of salmonella flagellin in vitro and in vivo

Formation of a flagellar filament of Salmonella from its component protein, flagellin, is in principle a self‐assembly process, which proceeds by the addition of flagellin monomers one by one to the distal end of the filament. Upon their polymerization, a conformational change of flagellin molecules occurs, and it confers polarity to the filament. For the initiation of in vitro flagellin assembly in a solution of physiological ionic strength and pH, it is essential to add fragments of flagellar filaments, which work as seeds for the polymerization of flagellin monomers. When an appropriate concentration of some anion known as a good salting‐outer is added to the solution, the polymerization begins without addition of seed. Assembly of flagellins in vivo begins at the distal end of each hook. The distal ends of the hooks on the cells of a flagellin‐less mutant were shown to initiate the assembly of exogenous flagellin in vitro, although the efficiency was not as high as that of the in vivo initiation. A flagellar filament elongates in vitro at a constant rate as long as a sufficient amount of flagellin is supplied, and the elongation terminates by an error occurring at the growing end of the filament. On the contrary, the rate of in vivo elongation decreases exponentially with increase of the length of the filament. Initial rate of the in vivo elongation depends on growth condition of the bacteria, while decrease of the rate per unit filament length is little affected by the growth condition. The observed limit in length of the flagellar filaments on growing bacteria is expected from the exponential decrease of their rate of elongation. The decrease of the in vivo elongation is correlated with the lowering of the transportation efficiency of flagellin monomers on their passage from the cell body through the central canal of a flagellar filament to the tip.