Novel insight into an energy transduction mechanism of the bacterial flagellar type III protein export

Type III secretion system (T3SS) is a protein translocator complex family including pathogenic injectisome or bacterial flagellum. The inejectisomal T3SS serves to deliver virulence proteins into host cell and the flagellar T3SS constructs the flagellar axial structure. Although earlier studies have provided many findings on the molecular mechanism of the Type III protein export, they were not sufficient to reveal energy transduction mechanism due to difficulties in controlling measurement conditions in vivo. Recently, we developed an in vitro flagellar Type III protein transport assay system using inverted membrane vesicles (IMVs), and analyzed protein export by using the in vitro method. We reproduced protein export of the flagellar T3SS, hook assembly and substrate specificity switch in IMV to a similar extent to what is seen in living cell. Furthermore, we demonstrated that ATP-hydrolysis energy can drive protein transport even in the absence of proton-motive force (PMF). In this mini-review, we will summarize our new in vitro Type III transport assay method and our findings on the molecular mechanism of Type III protein export.