Cwc24p, a NovelSaccharomyces cerevisiaeNuclear Ring Finger Protein, Affects Pre-snoRNA U3 Splicing
Journal of Biological Chemistry
U3 snoRNA is transcribed from two intron-containing genes in yeast, snR17A and snR17B. Although the assembly of the U3 snoRNP has not been precisely determined, at least some of the core box C/D proteins are known to bind pre-U3 co-transcriptionally, thereby affecting splicing and 3-end processing of this snoRNA. We identified the interaction between the box C/D assembly factor Nop17p and Cwc24p, a novel yeast RING finger protein that had been previously isolated in a complex with the splicing
... actor Cef1p. Here we show that, consistent with the protein interaction data, Cwc24p localizes to the cell nucleus, and its depletion leads to the accumulation of both U3 pre-snoRNAs. U3 snoRNA is involved in the early cleavages of 35 S pre-rRNA, and the defective splicing of pre-U3 detected in cells depleted of Cwc24p causes the accumulation of the 35 S precursor rRNA. These results led us to the conclusion that Cwc24p is involved in pre-U3 snoRNA splicing, indirectly affecting pre-rRNA processing. In eukaryotes, three of the rRNAs (18, 5.8, and 25 S in the yeast Saccharomyces cerevisiae) are transcribed by RNA pol 3 I as a single precursor, which undergoes various processing steps that include nucleotide modifications and endo-and exonucleolytic cleavages. In yeast, at least 150 factors are involved in 35 S pre-rRNA processing, including different proteins and snoRNPs. Processing and modification of the pre-rRNA seems to occur simultaneously with the assembly of ribosomal proteins, and large transient ribonucleoprotein (RNP) particles are formed in the nucleolus (1-3). Different sets of proteins were found in each complex. Among the proteins identified in the 90 S preribosomal complex are many of the small subunit ribosomal proteins, the core box C/D proteins (Nop1p, Nop56p, and Nop58p), U3 snoRNP-specific proteins, and other factors that might also be involved in the early steps of pre-rRNA processing (2). Proteins present in the pre-60 S complex are mainly involved in processing of 5.8 S and 25 S and in the formation of the large ribosome subunit (4, 5).