The crystal structure of the AF2331 protein from Archaeoglobus fulgidus DSM 4304 forms an unusual interdigitated dimer with a new type of alpha + beta fold.

The structure of AF2331, a 11-kDa orphan protein of unknown function from Archaeoglobus fulgidus, was solved by Se-Met MAD to 2.4 A resolution. The structure consists of an alpha + beta fold formed by an unusual homodimer, where the two core beta-sheets are interdigitated, containing strands alternating from both subunits. The decrease in solvent-accessible surface area upon dimerization is unusually large (3960 A(2)) for a protein of its size. The percentage of the total surface area buried in the interface (41.1%) is one of the largest observed in a nonredundant set of homodimers in the PDB and is above the mean for nearly all other types of homo-oligomers. AF2331 has no sequence homologs, and no structure similar to AF2331 could be found in the PDB using the CE, TM-align, DALI, or SSM packages. The protein has been identified in Pfam 23.0 as the archetype of a new superfamily and is topologically dissimilar to all other proteins with the "3-Layer (BBA) Sandwich" fold in CATH. Therefore, we propose that AF2331 forms a novel alpha + beta fold. AF2331 contains multiple negatively charged surface clusters and is located on the same operon as the basic protein AF2330. We hypothesize that AF2331 and AF2330 may form a charge-stabilized complex in vivo, though the role of the negatively charged surface clusters is not clear.

An extremely SAD case: structure of a putative redox-enzyme maturation protein from Archaeoglobus fulgidus at 3.4 A resolution.

This paper describes the crystal structure of AF0173, a putative redox-enzyme maturation protein (REMP) from Archaeoglobus fulgidus. The REMPs serve as chaperones in the maturation of extracytoplasmic oxidoreductases in archaea and bacteria. The all-helical subunits of AF0173 form a dimer arising from the interaction of residues located in a funnel-shaped cavity on one subunit surface with an uncut expression tag from the other subunit. This cavity is likely to represent a binding site for the twin-arginine motif that interacts with REMPs. The conservation of the overall fold in AF0173 and bacterial REMPs as well as the presence of conserved residues in their putative binding sites indicates that REMPs act in a similar manner in archaea and bacteria despite their limited sequence similarity. A model of the binding of the twin-arginine motif by AF0173 is suggested. The solution of the AF0173 structure by the single anomalous dispersion method represents an extreme case of SAD structure determination: low resolution (3.4 A), the absence of NCS and the presence of only two anomalously scattering atoms in the asymmetric unit. An unusually high solvent content (73%) turned out to be important for the success of the density-modification procedures.