Proteins of the Imp4/Brix superfamily are involved in ribosomal RNA processing, an essential function in all cells. We report the first structure of an Imp4/Brix superfamily protein, the Mil (for Methanothermobacter thermautotrophicus Imp4‐like) protein (gene product Mth680), from the archaeon M. thermautotrophicus. The amino‐ and carboxy‐terminal halves of Mil show significant structural similarity to one another, suggesting an origin by means of an ancestral duplication. Both halves show the same fold as the anticodon‐binding domain of class IIa aminoacyl‐tRNA synthetases, with greater conservation seen in the N‐terminal half. This structural similarity, together with the charge distribution in Mil, suggests that Imp4/Brix superfamily proteins could bind single‐stranded segments of RNA along a concave surface formed by the N‐terminal half of their β‐sheet and a central α‐helix. The crystal structure of Mil is incompatible with the presence, in the Imp4/Brix domain, of a helix–turn–helix motif that was proposed to comprise the RNA‐binding moiety of the Imp4/Brix proteins.
- Received October 6, 2004.
- Revision received November 8, 2004.
- Accepted December 3, 2004.
- Copyright © 2005 European Molecular Biology Organization