Structural characterization of FlgE2 protein from Helicobacter pylori hook

Helicobacter pylori flagellum is a complex rotatory nano-machine fundamental for bacterium survival in the human stomach. Protein FlgE is a component of the hook, a flexible junction exposed on the cell surface. In H. pylori genome two different genes are present in different positions coding for hypothetical FlgE. The first protein, FlgE1, is the actual component of the flagellum hook, whilst the second, FlgE2, shares only 26% of sequence identity with the other and its physiological function is still undefined. We have cloned, purified and crystallized FlgE2, whose structure, determined by the SAD method, shows that the overall structural organization of the protein is composed of three distinct domains, two of them relatively similar to that of other FlgE from other Gram-negative bacteria, whilst the third is quite peculiar to H. pylori. The crystal structure, along with the detected interaction with the regulatory cap protein FlgD, suggests a complementary function of FlgE1 and FlgE2 in H. pylori flagellum, possibly typical of polar flagella, confirming the role of both proteins in the flagellar hook organization. Although some general features are shared with other Gram-negative bacteria, the presence of two different hook proteins implicates that the molecular organization of H. pylori flagellum has its own peculiarities.