Although the U3 small nucleolar RNA (snoRNA), a member of the box C/D class of snoRNAs, was identified with the spliceosomal small nuclear RNAs (snRNAs) over 30 years ago, its function and its associated protein components have remained more elusive. The U3 snoRNA is ubiquitous in eukaryotes and is required for nucleolar processing of pre-18S ribosomal RNA in all organisms where it has been tested. Biochemical and genetic analyses suggest that U3 pre-rRNA base-pairing interactions mediate endonucleolytic pre-rRNA cleavages. Here we have purified a large ribonucleoprotein (RNP) complex from Saccharomyces cerevisiae that contains the U3 snoRNA and 28 proteins. Seventeen new proteins (Utp1 17) and Rrp5 were present, as were ten known components. The Utp proteins are nucleolar and specifically associated with the U3 snoRNA. Depletion of the Utp proteins impedes production of the 18S rRNA, indicating that they are part of the active pre-rRNA processing complex. On the basis of its large size (80S; calculated relative molecular mass of at least 2,200,000) and function, this complex may correspond to the terminal knobs present at the 5' ends of nascent pre-rRNAs. We have termed this large RNP the small subunit (SSU) processome.

We have isolated and characterized Mpp10p, a novel protein component of the U3 small nucleolar ribonucleoprotein (snoRNP) from the yeast Saccharomyces cerevisiae. The MPP10 protein was first identified in human cells by its reactivity with an antibody that recognizes specific sites of mitotic phosphorylation. To study the functional role of MPP10 in pre-rRNA processing, we identified the yeast protein by performing a GenBank search. The yeast Mpp10p homolog is 30% identical to the human protein over its length. Antibodies to the purified yeast protein recognize a 110-kDa polypeptide in yeast extracts and immunoprecipitate the U3 snoRNA, indicating that Mpp10p is a specific protein component of the U3 snoRNP in yeast. As a first step in the genetic analysis of Mpp10p function, diploid S. cerevisiae cells were transformed with a null allele. Sporulation and tetrad analysis indicate that MPP10 is an essential gene. A strain was constructed where Mpp10p is expressed from a galactose-inducible, glucose-repressible promoter. After depletion of Mpp10p by growth in glucose, cell growth is arrested and levels of 18S and its 20S precursor are reduced or absent while the 23S and 35S precursors accumulate. This pattern of accumulation of rRNA precursors suggests that Mpp10p is required for cleavage at sites A0, A1, and A2. Pulse-chase analysis of newly synthesized pre-rRNAs in Mpp10p-depleted yeast confirms that little mature 18S rRNA formed. These results reveal a novel protein essential for ribosome biogenesis and further elucidate the composition of the U3 snoRNP.In all eukaryotes, rRNA is transcribed as a single long transcript and processed by cleavages, nucleotide modification, and exonucleolytic degradation to generate the mature rRNAs. In the yeast Saccharomyces cerevisiae, these reactions result in the production of the mature 18S, 5.8S, and 25S rRNAs, which are assembled with the 5S ribonucleoprotein (RNP) and ribosomal proteins to form mature ribosomes. These events take place in the cell nucleolus. A number of small nucleolar ribonucleoproteins (snoRNPs) are required for many of these processing steps (25,27,40,45).Because it was readily identified in both vertebrate and yeast cells, the U3 snoRNP has been studied in a number of different organisms. Functional studies on the role of the U3 snoRNP in pre-rRNA processing have been carried out with cell extracts from mouse cells and Xenopus oocytes and in vivo in Xenopus laevis oocytes and in S. cerevisiae (4,6,18,22,30,34). Collectively, the results from these experiments point to an obligate role for the U3 snoRNP in the cleavages in the 5Ј external transcribed spacer (ETS) and in internal transcribed spacer 1 (ITS1) that generate the 18S rRNA (A0, A1, and A2 in Fig. 6).The 5Ј-most U3-dependent cleavage site (A0) in yeast also requires the presence of the RNase III homolog, the Rnt1 protein (10). In fact, in vitro yeast RNase III will cleave at this site in the absence of any other factors. This suggests that RNase III is catalytic for this processing step and th...

The rat I~-tropomyosin (13-TM) gene encodes both skeletal muscle 13-TM and fibroblast TM-1 by an alternative RNA-splicing mechanism. This gene contains 11 exons. Exons 1-5, 8, and 9 are common to all mRNAs expressed from the gene. Exons 6 and 11 are used in fibroblasts as well as smooth muscle cells, whereas exons 7 and 10 are used in skeletal muscle cells. In this study we have carried out an extensive mutational analysis to identify c/s-acting elements that block the use of the skeletal muscle-specific exon 7 in nonmuscle cells. These studies localize the critical elements for regulated alternative splicing to sequences within exon 7 and the adjacent upstream intron. In addition, mutations that inactivate the 5'-or 3'-splice sites of exon 6 do not result in the use of the skeletal muscle-specific exon 7 in nonmuscle cells, suggesting that splice-site selection in vivo is not regulated by a simple c/s-acting competition mechanism but, rather, by a mechanism that inhibits the use of exon 7 in certain cellular environments. In support of this hypothesis we have identified sequence-specific RNA-binding proteins in HeLa cell nuclear extracts using native gel electrophoresis and binding competition assays. Mutations in the pre-mRNA that result in the use of the skeletal muscle exon in vivo also disrupt the binding of these proteins to the RNA in vitro. We propose that the binding of these proteins to the pre-mRNA is involved in regulated alternative splicing and that this interaction is required for blocking the use of the skeletal muscle exon in nonmuscle cells.

We sought to identify all genes in the Candida albicans genome database whose deduced proteins would likely be soluble secreted proteins (the secretome). While certain C. albicans secretory proteins have been studied in detail, more data on the entire secretome is needed. One approach to rapidly predict the functions of an entire proteome is to utilize genomic database information and prediction algorithms. Thus, we used a set of prediction algorithms to computationally define a potential C. albicans secretome. We first assembled a validation set of 47 C. albicans proteins that are known to be secreted and 47 that are known not to be secreted. The presence or absence of an N-terminal signal peptide was correctly predicted by SignalP version 2.0 in 47 of 47 known secreted proteins and in 47 of 47 known nonsecreted proteins. When all 6165 C. albicans ORFs from CandidaDB were analysed with SignalP, 495 ORFs were predicted to encode proteins with N-terminal signal peptides. In the set of 495 deduced proteins with N-terminal signal peptides, 350 were predicted to have no transmembrane domains (or a single transmembrane domain at the extreme N-terminus) and 300 of these were predicted not to be GPI-anchored. TargetP was used to eliminate proteins with mitochondrial targeting signals, and the final computationally-predicted C. albicans secretome was estimated to consist of up to 283 ORFs. The C. albicans secretome database is available at http://info.med.yale.edu/intmed/infdis/candida/

We have previously developed a novel technique for isolation of cDNAs encoding M phase phosphoproteins (MPPs). In the work described herein, we further characterize MPP10, one of 10 novel proteins that we identified, with regard to its potential nucleolar function. We show that by cell fractionation, almost all MPP10 was found in isolated nucleoli. By immunofluorescence, MPP10 colocalized with nucleolar fibrillarin and other known nucleolar proteins in interphase cells but was not detected in the coiled bodies stained for either fibrillarin or p80 coilin, a protein found only in the coiled body. When nucleoli were separated into fibrillar and granular domains by treatment with actinomycin D, almost all the MPP10 was found in the fibrillar caps, which contain proteins involved in rRNA processing. In early to middle M phase of the cell cycle, MPP10 colocalized with fibrillarin to chromosome surfaces. At telophase, MPP10 was found in cellular structures that resembled nucleolus-derived bodies and prenucleolar bodies. Some of these bodies lacked fibrillarin, a previously described component of nucleolus-derived bodies and prenucleolar bodies, however, and the bulk of MPP10 arrived at the nucleolus later than fibrillarin. To further examine the properties of MPP10, we immunoprecipitated it from cell sonicates. The resulting precipitates contained U3 small nucleolar RNA (snoRNA) but no significant amounts of other box C/D snoRNAs. This association of MPP10 with U3 snoRNA was stable to 400 mM salt and suggested that MPP10 is a component of the human U3 small nucleolar ribonucleoprotein.