Enteroaggregative Escherichia coli is the primary cause of pediatric diarrhea in
developing countries and utilize aggregative adherence fimbriae (AAFs) to promote
initial adherence to the host intestinal mucosa, promote the formation of biofilms and
mediate host invasion. Five AAFs have been identified to date and AAF/IV is amongst
the most prevalent found in clinical isolates. Here we present the X-ray crystal structure
of the AAF/IV tip protein HdaB at 2.0 Å resolution. It shares high structural homology
with members of the Afa/Dr superfamily of fimbriae, which are involved in host
invasion. We highlight surface exposed residues that share sequence homology and
propose that these may function in invasion and also non-conserved regions that could
mediate HdaB specific adhesive functions.