This family represents the U3 small nucleolar ribonucleoprotein protein IMP4 and Ribosome production factor 1 RPF1 that belong to the IMP4 superfamily. This superfamily contains a Brix (biogenesis of ribosomes in Xenopus) domain, a region of 150-180 residues length, and it is characterised by the sigma70-like motif, a sequence motif first identified in the sigma70 family of prokaryotic transcription factors, which is sufficient to confer RNA binding activity [, , ]. IMP4 is a component of the 60-80S U3 small nucleolar ribonucleoprotein (U3 snoRNP) and it is required for the early cleavages during pre-18S ribosomal RNA processing [, ]. RPF1 is a essential protein involved in ribosome biogenesis [].
Analysis of the Brix (biogenesis of ribosomes in Xenopus) protein leaded to the identification of a region of 150-180 residues length, called the Brixdomain, which is found in six protein families: one archaean family (I) including hypothetical proteins (one per genome); and five eukaryote families, each named according to a representative member and including close homologues of this prototype: (II) Peter Pan (D. melanogaster) and SSF1/2 (S.cerevisiae); (III) RPF1 (S. cerevisiae); (IV) IMP4 (S. cerevisiae); (V) Brix (X.laevis) and BRX1 (S. cerevisiae); and (VI) RPF2 (S.cerevisiae).Typically, a protein sequence belonging to the Brix domain superfamily contains a highly charged N-terminal segment (about 50 residues) followed by a single copy of the Brix domain and another highly charged C-terminal region (about 100 residues). The archaean sequences have two unique characteristics: (1) the charged regions are totally absent at the N terminus and are reduced in number to about 10 residues at the C terminus; and (2) the C-terminal part of the Brix domain itself is minimal. Two eukaryote groups have large insertions within the C-terminal region: about 70 residues in the group III and about 120 in the group II. Biological data for some proteins in this family suggest a role in ribosome biogenesis and rRNA binding [, , , ].
