Brix from xenopus laevis and brx1p from yeast define a new family of proteins involved in the biogenesis of large ribosomal subunits

A clone was isolated from a cDNA library from early embryos of Xenopus laevis that codes for a highly charged protein containing 339 amino acids. Two putative nuclear localization signals could be identified in its sequence, but no other known motifs or domains. Closely related ORFs are present in the genomes of man, C. elegans, yeast and Arabidopsis. A fusion protein with GFP expressed in HeLa cells or Xenopus oocytes was found to be localized in the nucleolus and coiled (Cajal) bodies. Moreover, immunoprecipitation experiments demonstrated that the new Xenopus protein interacts with 5S, 5.8S and 28S RNAs of large ribosomal subunits. The name Brix (biogenesis of ribosomes in Xenopus) is proposed for this protein and the corresponding gene. In Saccharomyces cerevisiae, the essential gene YOL077c, now named BRX1, codes for the Brix homolog, which is also localized in the nucleolus. Depletion of Brx1 p in a conditional yeast mutant leads to defects in rRNA processing, and a block in the assembly of large ribosomal subunits.