<p>This entry represents the core region of several hypothetical proteins found in bacteria, plants, and yeast proteins. This core region can be subdivided into three domains: a 3-helical bundle domain, and two alpha+beta domains with different folds, where domain 3 (ferredoxin-like fold) is inserted within domain 2. This core region is found in the following hypothetical proteins: YebC from <taxon tax_id="562">Escherichia coli</taxon>, HP0162 from <taxon tax_id="210">Helicobacter pylori</taxon> (Campylobacter pylori) and aq1575 from <taxon tax_id="63363">Aquifex aeolicus</taxon> [<cite idref="PUB00027019"/>].</p><p>The crystal structure of a conserved hypothetical protein, Aq1575, from <taxon tax_id="63363">Aquifex aeolicus</taxon> has been determined. A structural homology search reveals that this protein has a new fold with no obvious similarity to those of other proteins of known three-dimensional structure. The protein reveals a monomer consisting of three domains arranged along a pseudo threefold symmetry axis. There is a large cleft with approximate dimensions of 10 A x 10 A x 20 A in the centre of the three domains along the symmetry axis. Two possible active sites are suggested based on the structure and multiple sequence alignment. There are several highly conserved residues in these putative active sites [<cite idref="PUB00027019"/>].</p>