Self-Oligomerizing Structure of the Flagellar Cap Protein FliD and Its Implication in Filament Assembly

초록

FliD is a self-oligomerizing structural protein that caps the growing end of the bacterial flagellar filament. FIiD also plays a key role in the flagellar system by continuously adding a new flagellin protein to the tip of the filament. To structurally characterize FliD oligomerization and to provide a FliD-mediated flagellin polymerization mechanism, we have determined the crystal structures of FIiD proteins from Escherichia coli and Salmonella enterica serovar Typhimurium (ecFliD and stFliD, respectively). ecFliD consists of three domains (D1, D2, and D3) and forms a hexamer plate of the D2 and D3 domains that resembles a six-pointed star with legs consisting of the D1 domain. In contrast, the D2 and D3 domains of stFliD assemble into a pentamer as a five-pointed star plate. Despite their distinct oligomeric states, ecFliD and stFliD engage a common molecular surface for oligomerization. FIiD also features interdomain and intersubunit flexibility, suggesting that FIiD reorganizes its domains and adjacent subunits depending on the FIiD binding partner. The similarity of the FIiD shape to flagellin and the structural dynamics of FliD led us to propose a FliD-catalyzed filament elongation mechanism. In this model, FIiD occupies a position in place of a nascent flagellin until the flagellin reaches the growing end of the filament, and then, FIiD moves aside to repeat the positional replacement. (C) 2017 Elsevier Ltd. All rights reserved.

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