Watkins et al., 1996; Samarsky et al., 1998). It is also clear that a fraction of snoRNAs, such as U3, U8, and U14, depend on the same sets of conserved box C/D sequences to carry out site-specific cleavage of ribo-somal RNAs (Baserga et al. Peculis and Steitz, 1994; Samarsky and Fournier, 1998; Watkins et al., 2002). Another type of snoRNP, the box H/ACA snoRNPs, catalyzes site-specific pseudouridyla-tion of rRNA by employing similar mechanisms of sub-Summary strate recognition and enzyme organization (Ofengand and Fournier, 1998; Venema and Tollervey, 1999; Kiss, We have determined and refined a crystal structure 2002; Rozhdestvensky et al., 2003). of the initial assembly complex of archaeal box C/D Earlier biochemical studies have shown that the con-sRNPs comprising the Archaeoglobus fulgidus (AF) served box C and D sequences are the sites of protein L7Ae protein and a box C/D RNA. The box C/D RNA assembly (Kuhn et al., 2002; Szewczak et al., 2002; Wat-forms a classical kink-turn (K-turn) structure and the kins et al., 2002). The first protein that interacts with the resulting protein-RNA complex serves as a distinct box C/D sequences is a member of an RNA binding platform for recruitment of the fibrillarin-Nop5p com-protein family. This protein was identified to be 15.5 kDa plex. The cocrystal structure confirms previously pro-(Watkins et al., 2000) in human and L7Ae in Archaea posed secondary structure of the box C/D RNA that (Kuhn et al., 2002). The initial interaction established includes a protruded U, a UU mismatch, and two between the RNA binding protein and the box C/D motif sheared tandem GA base pairs. Detailed structural is required for subsequent assembly of other snoRNP or comparisons of the AF L7Ae-box C/D RNA complex sRNP proteins, including fibrillarin, the eucaryal Nop56/ with previously determined crystal structures of L7Ae 58p, or the archaeal Nop5p proteins (Watkins et al., homologs in complex with functionally distinct K-turn 2002; Cahill et al., 2002; Rashid et al., 2003; Tran et al., RNAs revealed a set of remarkably conserved princi-2003). Although the Nop56/Nop5p proteins appear to ples in protein-RNA interactions. These analyses pro-interact directly with the conserved box C/D elements vide a structural basis for interpreting the functional upon binding of the 15.5 kDa/L7Ae protein (Cahill et al., roles of the box C/D sequences in directing specific 2002; Rashid et al., 2003; Watkins et al., 2002), fibrillarin assembly of box C/D sRNPs. is the subunit that catalyzes the actual methyl transfer reaction (Niewmierzycka and Clarke, 1999; Omer et al., Introduction 2000, 2002; Wang et al., 2000). Thus the recruitment of fibrillarin in the assembly of box C/D s(no)RNPs medi-Box C/D small nucleolar ribonucleoprotein particles ated by the 15.5 kDa/L7Ae protein suggests an impor-(snoRNPs) catalyze site-specific 2-O-methylation and tant functional role for 15.5 kDa/L7Ae in constructing the processing of ribosomal RNAs (rRNAs). These reactions RNA architecture around the active site before catal...
