The crystal structure of PD1, a Haemophilus surface fibril domain

The Haemophilus Surface Fibril (Hsf) is an unusually large trimeric autotransporter adhesin (TAA) expressed by the most virulent strains of Haemophilus influenzae. Hsf is known to mediate adhesion between pathogen and host, allowing the establishment of potentially deadly diseases such as epiglottitis, meningitis, and pneumonia. While recent research suggested that this TAA might adopt a novel “hairpin-like” architecture, the characterisation of Hsf has been limited to in silico modelling and electron micrographs with no high-resolution structural data available. Here, we report the crystal structure of Hsf putative domain 1, PD1, at 3.3 Å. The structure corrects previous domain annotation by revealing the presence of an unexpected N-terminal TrpRing domain. PD1 represents the first Hsf domain to be solved, and thus paves the way for further research on the “hairpin-like” hypothesis.