This family of proteins is found exclusively in epsilon-proteobacteria. Proteins in this family are approximately 180 amino acids in length. The crystal structure of HobA from Helicobacter pylori has been reported at 1.7A resolution; HobA represents a modified Rossmann fold consisting of a five-stranded parallel β-sheet (beta1-5) flanked on one side by alpha-2, alpha-3 and alpha-6 helices and alpha-4 and alpha-5 on the other. The alpha-1 helix is extended away from and has minimal interaction with the globular part of the protein. Four monomers interact to form a tetrameric molecule. Four calcium atoms bind to the tetramer and these binding sites may have functional relevance. The closest structural homologue of HobA is a sugar isomerase (SIS) domain containing protein, the phosphoheptose isomerase from Pseudomonas aeruginosa. The SIS proteins share strong sequence homology with DiaA from Escherichia coli; yet, HobA and DiaA share no sequence homology []. HobA is a novel protein essential for initiation of H. pylori chromosome replication. It interacts specifically via DnaA with the oriC-DnaA complex. It is possible that HobA is essential for the correct formation and stabilisation of the orisome by facilitating the spatial positioning of DnaA at oriC [].
