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.