Structural modeling of the flagellum MS ring protein FliF reveals similarities to the type III secretion system and sporulation complex

Abstract

The flagellum is a large proteinaceous organelle found at the surface of many bacteria, whose primary role is to allow motility through the rotation of a long extracellular filament. It is an essential virulence factor in many pathogenic species, and is also a priming component in the formation of antibiotic-resistant biofilms. The flagellum consists of the export apparatus and stator in the cytosol; the basal body, spanning the bacterial membrane(s) and periplasm; and the hook-filament, that protrudes away from the bacterial surface. Assembly of the bacterial flagellum is initiated by the formation of the basal body MS ring region, constituted of multiple copies of the protein FliF. Here, I report an analysis of the FliF sequence from various bacterial species, demonstrating that its periplasmic region is composed of a domain homologuous to that of the type III secretion system proteins PrgK, and of a second globular domain that possesses a similar fold to that of the sporulation complex component SpoIIIAG. I also describe that Chlamydia possesses an unusual FliF protein, lacking part of the PrgK homology domain and the SpoIIIAG-like domain, and fused to FliG at its C-terminus. Finally, I have combined the sequence analysis of FliF with the EM map of the MS ring, to propose the first atomic model for the FliF oligomer. These results further emphasize the similarity between the flagellum, T3SS and sporulation complex, and will facilitate further structural studies.