Role of Export Chaperones in Regulation of Flagellum Assembly

Abstract

The bacterial flagellum is one of the most sophisticated nano-machines in nature. The majority of flagellum stays outside the cell exterior, therefore, to build flagellum bacteria develops a dedicated flagellar export system to transport flagellar proteins to the distal growing end of a flagellum. Among the key export associated proteins, there are flagellar chaperones which stabilize the export substrates in the cytoplasm, prevents their undesired interaction, and targets them to the export gate. Flagellum assembly is strictly regulated, and export chaperones play crucial roles in regulating export order. FliT, FliS, FlgN are export chaperones for export substrates, FliD, FliC, FlgK, and FlgL, respectively. FliJ is a general chaperone that assists and regulates the export process. FlhA and FlhB constitute the export gate and determines the hierarchy of the export process. Despite detailed understanding of the morphology of flagellum, flagella export process is poorly understood. We have determined the solution structure of FliT in its apo form along with its complex with FliD, FliI, and FliJ. Solution structure of FliT differs from its crystal structure and explains the regulatory role of FliT. FliT stays in an auto-inhibitory form in the absence of substrates to avoid any unwanted interaction with export gate. Substrate binding activates FliT-substrate complex to bind to the export gate. This targeting mechanism of FliT is appeared to be shared among all export chaperones, FlgN and FliS. We further characterized the interactions among export chaperones and showed that strict binding sequence among chaperones determines chaperone recycling process and regulate subsequent export events.