The Extra-Membranous Domains of the Competence Protein HofQ Show DNA Binding, Flexibility and a Shared Fold with Type I KH Domains

Abstract

Secretins form large oligomeric assemblies in the membrane that control both macromolecular secretion and uptake. Several <em>Pasteurellaceae</em> are naturally competent for transformation, but the mechanism for DNA assimilation is largely unknown. In <em>Haemophilus influenzae</em>, the secretin ComE has been demonstrated to be essential for DNA uptake. In closely related <em>Aggregatibacter actinomycetemcomitans</em>, an opportunistic pathogen in periodontitis, the ComE homolog HofQ is believed to be the outer membrane DNA translocase. Here, we report the structure of the extramembranous domains of HofQ at 2.3 å resolution by X-ray crystallography. We also show that the extra-membranous domains of HofQ are capable of DNA binding. The structure reveals two secretin-like folds, the first of which is formed by means of a domain swap. The second domain displays extensive structural similarity to K homology (KH) domains, including the presence of a GxxG motif, which is essential for the nucleotide-binding function of KH domains, suggesting a possible mechanism for DNA binding by HofQ. The data indicate a direct involvement in DNA acquisition and provide insight into the molecular basis for natural competence. <br />