The bacterial flagellum is a filamentous organelle extending from the cell 
surface. The axial structure of the flagellum consists of the rod, hook, 
junction, filament, and cap. The axial structure is formed by axial component 
proteins exported via a specific protein export apparatus in a well-regulated 
manner. Although previous studies have revealed the outline of the flagellar 
construction process, the mechanism of axial structure formation, including 
axial protein export, is still obscure due to difficulties in direct observation 
of protein export and assembly in vivo. We recently developed an in vitro 
flagellar protein transport assay system using inverted membrane vesicles (IMVs) 
and succeeded in reproducing the early stage of flagellar assembly. However, the 
late stage of the flagellar formation process remained to be examined in the 
IMVs. In this study, we showed that the filament-type proteins are transported 
into the IMVs to produce the filament on the hook inside the IMVs. Furthermore, 
we provide direct evidence that coordinated flagellar protein export and 
assembly can occur at the post-translational level. These results indicate that 
the ordered construction of the entire flagellar structure can be regulated by 
only the interactions between the protein export apparatus, the export substrate 
proteins, and their cognate chaperones.