HadA is an atypical new multifunctional trimeric coiled-coil adhesin of Haemophilus influenzae biogroup aegyptius, which promotes entry into host cells.

The Oca (Oligomeric coiled-coil adhesin) family is a subgroup of the bacterial trimeric autotransporter adhesins, which includes structurally related proteins, such as YadA of Yersinia enterocolitica and NadA of Neisseria meningitidis. In this study, we searched in silico for novel members of this family in bacterial genomes and identified HadA (Haemophilus adhesin A), a trimeric autotransporter expressed only by Haemophilus influenzae biogroup aegyptius causing Brazilian purpuric fever (BPF), a fulminant septicemic disease of children. By comparative genomics and sequence analysis we predicted that the hadA gene is harboured on a mobile genetic element unique to BPF isolates. Biological analysis of HadA in the native background was limited because this organism is not amenable to genetic manipulation. Alternatively, we demonstrated that expression of HadA confers to a non-invasive Escherichia coli strain the ability to adhere to human cells and to extracellular matrix proteins and to induce in vitro bacterial aggregation and microcolony formation. Intriguingly, HadA is predicted to lack the typical N-terminal head domain of Oca proteins generally associated with cellular receptor binding. We propose here a structural model of the HadA coiled-coil stalk and show that the N-terminal region is still responsible of the binding activity and a KGD motif plays a role. Interestingly, HadA promotes bacterial entry into mammalian cells. Our results show a cytoskeleton re-arrangement and an involvement of clathrin in the HadA-mediated internalization. These data give new insights on the structure-function relationship of oligomeric coiled-coil adhesins and suggest a potential role of this protein in the pathogenesis of BPF.