Molecular Determinants of Filament Capping Proteins Required for the Formation of Functional Flagella in Gram-Negative Bacteria.

Sandra Postel,Eric J Sundberg,Brian G Pierce,Marko Nedeljković

doi:10.3390/biom11101397

Abstract

Bacterial flagella are cell surface protein appendages that are critical for motility and pathogenesis. Flagellar filaments are tubular structures constructed from thousands of copies of the protein flagellin, or FliC, arranged in helical fashion. Individual unfolded FliC subunits traverse the filament pore and are folded and sorted into place with the assistance of the flagellar capping protein complex, an oligomer of the FliD protein. The FliD filament cap is a stool-like structure, with its D2 and D3 domains forming a flat head region, and its D1 domain leg-like structures extending perpendicularly from the head towards the inner core of the filament. Here, using an approach combining bacterial genetics, motility assays, electron microscopy and molecular modeling, we define, in numerous Gram-negative bacteria, which regions of FliD are critical for interaction with FliC subunits and result in the formation of functional flagella. Our data indicate that the D1 domain of FliD is its sole functionally important domain, and that its flexible coiled coil region comprised of helices at its extreme N- and C-termini controls compatibility with the FliC filament. FliD sequences from different bacterial species in the head region are well tolerated. Additionally, head domains can be replaced by small peptides and larger head domains from different species and still produce functional flagella.

Highlights

Pathogenic bacteria that cause a multitude of diseases in humans often rely on flagellabased motility through liquid environments to reach and adhere to host target cells, making bacterial flagella nearly indispensable for effective infection and colonization [1,2]
The flagellar filament is composed of thousands of copies of the protein flagellin, or FliC, ordered in a helical manner around a central pore with a capping protein complex composed of FliD protein subunits that form a chamber at the distal end of the filament
Individual unfolded subunits of FliC traverse through the hollow filament pore from the bacterial cytoplasm prior to being folded and sorted into position at the tip of the growing filament [3]

Summary

Introduction

Pathogenic bacteria that cause a multitude of diseases in humans often rely on flagellabased motility through liquid environments to reach and adhere to host target cells, making bacterial flagella nearly indispensable for effective infection and colonization [1,2]. The flagellar filament is composed of thousands of copies of the protein flagellin, or FliC, ordered in a helical manner around a central pore with a capping protein complex composed of FliD protein subunits that form a chamber at the distal end of the filament. Individual unfolded subunits of FliC traverse through the hollow filament pore from the bacterial cytoplasm prior to being folded and sorted into position at the tip of the growing filament [3]. The exact role of the FliD cap in FliC folding and sorting is not fully understood. In bacteria that do not express FliD, flagellar filaments are not formed, resulting in impaired motility and infectivity.

Methods

Results

Conclusion