Fibrillarin and Nop56 interact before being co-assembled in box C/D snoRNPs

Small nucleolar RNAs play crucial roles in ribosome biogenesis. They guide folding, site-specific nucleotide modifications and participate in cleavage of precursor ribosomal RNAs. To better understand how the biogenesis of the box CID small nucleolar RNPs (snoRNPs) occur in a cellular context, we used a new approach based on the possibility of relocalizing a given nuclear complex by adding an affinity tag for B23 to one component of this complex. We selectively delocalized each core box CID protein, namely 15.5kD, Nop56, Nop58 and fibrillarin, and analyzed the effect of such changes on other components of the box CID snoRNPs. We show that modifying the localization and the mobility of core box CID proteins impairs their association with box CID snoRNPs. In addition, we demonstrate that fibrillarin and Nop56 directly interact in vivo. This interaction, indispensable for the association of both proteins with the box CID snoRNPs, does not involve the glycine- and arginine-rich domain or the RNA-binding domain but the alpha-helix domain of fibrillarin. In addition, no RNA seems required to maintain fibrillarin-Nop56 interaction. (C) 2009 Elsevier Inc. All rights reserved