Nucleocytoplasmic transport and snRNP assembly

Immobile survival of motoneuron (SMN) protein stored in Cajal bodies can be mobilized by protein interactions

Reduced levels of survival of motoneuron (SMN) protein lead to spinal muscular atrophy, but it is still unknown how SMN protects motoneurons in the spinal cord against degeneration. In the nucleus, SMN is associated with two types of nuclear bodies denoted as gems and Cajal bodies (CBs). The 23 kDa isoform of fibroblast growth factor-2 (FGF-2(23)) is a nuclear protein that binds to SMN and destabilizes the SMN-Gemin2 complex. In the present study, we show that FGF-2(23) depletes SMN from CBs without affecting their general structure. FRAP analysis of SMN-EGFP in CBs demonstrated that the majority of SMN in CBs remained mobile and allowed quantification of fast, slow and immobile nuclear SMN populations. The potential for SMN release was confirmed by in vivo photoconversion of SMN-Dendra2, indicating that CBs concentrate immobile SMN that could have a specialized function in CBs. FGF-2(23) accelerated SMN release from CBs, accompanied by a conversion of immobile SMN into a mobile population. Furthermore, FGF-2(23) caused snRNP accumulation in CBs. We propose a model in which Cajal bodies store immobile SMN that can be mobilized by its nuclear interaction partner FGF-2(23), leading to U4 snRNP accumulation in CBs, indicating a role for immobile SMN in tri-snRNP assembly.

Family size and turnover rates among several classes of small non-protein-coding RNA genes in Caenorhabditis nematodes

It is important to understand the forces that shape the size and evolutionary histories of gene families. Here, we investigated the evolution of non–protein-coding RNA genes in the genomes of Caenorhabditis nematodes. We specifically focused on nested arrangements, that is, cases in which an RNA gene is entirely contained in an intron of another gene. Comparing these arrangements between species simplifies the inference of orthology and, therefore, of evolutionary fates of nested genes. Two distinct patterns are evident in the data. Genes encoding small nuclear RNAs (snRNAs) and transfer RNAs form large families, which have persisted since before the common ancestor of Metazoa. Yet, individual genes die relatively rapidly, with few orthologs having survived since the divergence of Caenorhabditis elegans and Caenorhabditis briggsae. In contrast, genes encoding small nucleolar RNAs (snoRNAs) are either single-copy or form small families. Individual snoRNAs turn over at a relatively slow rate—most C. elegans genes have clearly identifiable orthologs in C. briggsae. We also found that in Drosophila, genes from larger snRNA families die at a faster rate than their counterparts from single-gene families. These results suggest that a relationship between family size and the rate of gene turnover may be a general feature of genome evolution.

Structure of a key intermediate of the SMN complex reveals Gemin2's crucial function in snRNP assembly

The SMN complex mediates the assembly of heptameric Sm protein rings on small nuclear RNAs (snRNAs), which are essential for snRNP function. Specific Sm core assembly depends on Sm proteins and snRNA recognition by SMN/Gemin2- and Gemin5-containing subunits, respectively. The mechanism by which the Sm proteins are gathered while preventing illicit Sm assembly on non-snRNAs is unknown. Here, we describe the 2.5 Å crystal structure of Gemin2 bound to SmD1/D2/F/E/G pentamer and SMN's Gemin2-binding domain, a key assembly intermediate. Remarkably, through its extended conformation, Gemin2 wraps around the crescent-shaped pentamer, interacting with all five Sm proteins, and gripping its bottom and top sides and outer perimeter. Gemin2 reaches into the RNA-binding pocket, preventing RNA binding. Interestingly, SMN-Gemin2 interaction is abrogated by a spinal muscular atrophy (SMA)-causing mutation in an SMN helix that mediates Gemin2 binding. These findings provide insight into SMN complex assembly and specificity, linking snRNP biogenesis and SMA pathogenesis.

RNA processing defects associated with diseases of the motor neuron

Rapid progress in the discovery of motor neuron disease genes in amyotrophic lateral sclerosis, the spinal muscular atrophies, hereditary motor neuropathies, and lethal congenital contracture syndromes is providing new perspectives and insights into the molecular pathogenesis of the motor neuron. Motor neuron disease genes are often expressed throughout the body with essential functions in all cells. A survey of these functions indicates that motor neurons are uniquely sensitive to perturbations in RNA processing pathways dependent on the interaction of specific RNAs with specific RNA-binding proteins, which presumably result in aberrant formation and function of ribonucleoprotein complexes. This review provides a summary of currently recognized RNA processing defects linked to human motor neuron diseases.

Molecular functions of the SMN complex

The SMN complex is essential for the biogenesis of spliceosomal small nuclear ribonucleoproteins and likely functions in the assembly, metabolism, and transport of a diverse number of other ribonucleoproteins. Specifically, the SMN complex assembles 7 Sm proteins into a core structure around a highly conserved sequence of ribonucleic acid (RNA) found in small nuclear RNAs. The complex recognizes specific sequences and structural features of small nuclear RNAs and Sm proteins and assembles small nuclear ribonucleoproteins in a stepwise fashion. In addition to the SMN protein, the SMN complex contains 7 additional proteins known as Gemin2-8, each likely to play a role in ribonucleoprotein biogenesis. This review focuses on the current understanding of the mechanism of the role of the SMN complex in small nuclear ribonucleoprotein assembly and considers the relationship of this function to spinal muscular atrophy.

Epstein-Barr virus noncoding RNAs are confined to the nucleus, whereas their partner, the human La protein, undergoes nucleocytoplasmic shuttling

The Epstein-Barr virus (EBV) noncoding RNAs, EBV-encoded RNA 1 (EBER1) and EBER2, are the most abundant viral transcripts in all types of latently infected human B cells, but their function remains unknown. We carried out heterokaryon assays using cells that endogenously produce EBERs to address their trafficking, as well as that of the La protein, because EBERs are quantitatively bound by La in vivo. Both in this assay and in oocyte microinjection assays, EBERs are confined to the nucleus, suggesting that their contribution to viral latency is purely nuclear. EBER1 does not bind exportin 5; therefore, it is unlikely to act by interfering with microRNA biogenesis. In contrast, La, which is a nuclear phosphoprotein, undergoes nucleocytoplasmic shuttling independent of the nuclear export protein Crm1. To ensure that small RNA shuttling can be detected in cells that are negative for EBER shuttling, we demonstrate the shuttling of U1 small nuclear RNA.

Specific sequence features, recognized by the SMN complex, identify snRNAs and determine their fate as snRNPs

The survival of motor neurons (SMN) complex is essential for the biogenesis of spliceosomal small nuclear ribonucleoproteins (snRNPs) as it binds to and delivers Sm proteins for assembly of Sm cores on the abundant small nuclear RNAs (snRNAs). Using the conserved snRNAs encoded by the lymphotropic Herpesvirus saimiri (HVS), we determined the specific sequence and structural features of RNAs for binding to the SMN complex and for Sm core assembly. We show that the minimal SMN complex-binding domain in snRNAs, except U1, is comprised of an Sm site (AUUUUUG) and an adjacent 3' stem-loop. The adenosine and the first and third uridines of the Sm site are particularly critical for binding of the SMN complex, which directly contacts the backbone phosphates of these uridines. The specific sequence of the adjacent stem (7 to 12 base pairs)-loop (4 to 17 nucleotides) is not important for SMN complex binding, but it must be located within a short distance of the 3' end of the RNA for an Sm core to assemble. Importantly, these defining characteristics are discerned by the SMN complex and not by the Sm proteins, which can bind to and assemble on an Sm site sequence alone. These findings demonstrate that the SMN complex is the identifier, as well as assembler, of the abundant class of snRNAs in cells because it is able to recognize an snRNP code that they contain.

The survival of motor neurons protein determines the capacity for snRNP assembly: biochemical deficiency in spinal muscular atrophy

Reduction of the survival of motor neurons (SMN) protein levels causes the motor neuron degenerative disease spinal muscular atrophy, the severity of which correlates with the extent of reduction in SMN. SMN, together with Gemins 2 to 7, forms a complex that functions in the assembly of small nuclear ribonucleoprotein particles (snRNPs). Complete depletion of the SMN complex from cell extracts abolishes snRNP assembly, the formation of heptameric Sm cores on snRNAs. However, what effect, if any, reduction of SMN protein levels, as occurs in spinal muscular atrophy patients, has on the capacity of cells to produce snRNPs is not known. To address this, we developed a sensitive and quantitative assay for snRNP assembly, the formation of high-salt- and heparin-resistant stable Sm cores, that is strictly dependent on the SMN complex. We show that the extent of Sm core assembly is directly proportional to the amount of SMN protein in cell extracts. Consistent with this, pulse-labeling experiments demonstrate a significant reduction in the rate of snRNP biogenesis in low-SMN cells. Furthermore, extracts of cells from spinal muscular atrophy patients have a lower capacity for snRNP assembly that corresponds directly to the reduced amount of SMN. Thus, SMN determines the capacity for snRNP biogenesis, and our findings provide evidence for a measurable deficiency in a biochemical activity in cells from patients with spinal muscular atrophy.

Why do cells need an assembly machine for RNA-protein complexes?

Small nuclear ribonucleoproteins (snRNPs) are crucial for pre-mRNA processing to mRNAs. Each snRNP contains a small nuclear RNA (snRNA) and an extremely stable core of seven Sm proteins. The snRNP biogenesis pathway is complex, involving nuclear export of snRNA, Sm-core assembly in the cytoplasm and re-import of the mature snRNP. Although in vitro Sm cores assemble readily on uridine-rich RNAs, the assembly in cells is carried out by the survival of motor neurons (SMN) complex. The SMN complex stringently scrutinizes RNAs for specific features that define them as snRNAs and identifies the RNA-binding Sm proteins. We discuss how this surveillance capacity of the SMN complex might ensure assembly of Sm cores only on the correct RNAs and prevent illicit, potentially deleterious assembly of Sm cores on random RNAs.

snRNAs contain specific SMN-binding domains that are essential for snRNP assembly

To serve in its function as an assembly machine for spliceosomal small nuclear ribonucleoprotein particles (snRNPs), the survival of motor neurons (SMN) protein complex binds directly to the Sm proteins and the U snRNAs. A specific domain unique to U1 snRNA, stem-loop 1 (SL1), is required for SMN complex binding and U1 snRNP Sm core assembly. Here, we show that each of the major spliceosomal U snRNAs (U2, U4, and U5), as well as the minor splicing pathway U11 snRNA, contains a domain to which the SMN complex binds directly and with remarkable affinity (low nanomolar concentration). The SMN-binding domains of the U snRNAs do not have any significant nucleotide sequence similarity yet they compete for binding to the SMN complex in a manner that suggests the presence of at least two binding sites. Furthermore, the SMN complex-binding domain and the Sm site are both necessary and sufficient for Sm core assembly and their relative positions are critical for snRNP assembly. These findings indicate that the SMN complex stringently scrutinizes RNAs for specific structural features that are not obvious from the sequence of the RNAs but are required for their identification as bona fide snRNAs. It is likely that this surveillance capacity of the SMN complex ensures assembly of Sm cores on the correct RNAs only and prevents illicit, potentially deleterious, assembly of Sm cores on random RNAs.

Xenopus LSm proteins bind U8 snoRNA via an internal evolutionarily conserved octamer sequence

U8 snoRNA plays a unique role in ribosome biogenesis: it is the only snoRNA essential for maturation of the large ribosomal subunit RNAs, 5.8S and 28S. To learn the mechanisms behind the in vivo role of U8 snoRNA, we have purified to near homogeneity and characterized a set of proteins responsible for the formation of a specific U8 RNA-binding complex. This 75-kDa complex is stable in the absence of added RNA and binds U8 with high specificity, requiring the conserved octamer sequence present in all U8 homologues. At least two proteins in this complex can be cross-linked directly to U8 RNA. We have identified the proteins as Xenopus homologues of the LSm (like Sm) proteins, which were previously reported to be involved in cytoplasmic degradation of mRNA and nuclear stabilization of U6 snRNA. We have identified LSm2, -3, -4, -6, -7, and -8 in our purified complex and found that this complex associates with U8 RNA in vivo. This purified complex can bind U6 snRNA in vitro but does not bind U3 or U14 snoRNA in vitro, demonstrating that the LSm complex specifically recognizes U8 RNA.

Sequence-specific interaction of U1 snRNA with the SMN complex

The survival of motor neurons (SMN) protein complex functions in the biogenesis of spliceosomal small nuclear ribonucleoprotein particles (snRNPs) and prob ably other RNPs. All spliceosomal snRNPs have a common core of seven Sm proteins. To mediate the assembly of snRNPs, the SMN complex must be able to bring together Sm proteins with U snRNAs. We showed previously that SMN and other components of the SMN complex interact directly with several Sm proteins. Here, we show that the SMN complex also interacts specifically with U1 snRNA. The stem--loop 1 domain of U1 (SL1) is necessary and sufficient for SMN complex binding in vivo and in vitro. Substitution of three nucleotides in the SL1 loop (SL1A3) abolishes SMN interaction, and the corresponding U1 snRNA (U1A3) is impaired in U1 snRNP biogenesis. Microinjection of excess SL1 but not SL1A3 into Xenopus oocytes inhibits SMN complex binding to U1 snRNA and U1 snRNP assembly. These findings indicate that SMN complex interaction with SL1 is sequence-specific and critical for U1 snRNP biogenesis, further supporting the direct role of the SMN complex in RNP biogenesis.

SMN interacts with a novel family of hnRNP and spliceosomal proteins

Spinal muscular atrophy (SMA) is a common neurodegenerative disease caused by deletion or loss-of-function mutations of the survival of motor neurons (SMN) protein. SMN is in a complex with several proteins, including Gemin2, Gemin3 and Gemin4, and it plays important roles in small nuclear ribonucleoprotein (snRNP) biogenesis and in pre-mRNA splicing. Here, we characterize three new hnRNP proteins, collectively referred to as hnRNP Qs, which are derived from alternative splicing of a single gene. The hnRNP Q proteins interact with SMN, and the most common SMN mutant found in SMA patients is defective in its interactions with them. We further demonstrate that hnRNP Qs are required for efficient pre-mRNA splicing in vitro. The hnRNP Q proteins may provide a molecular link between the SMN complex and splicing.

A Sm-like protein complex that participates in mRNA degradation

In eukaryotes, seven Sm proteins bind to the U1, U2, U4 and U5 spliceosomal snRNAs while seven Smlike proteins (Lsm2p-Lsm8p) are associated with U6 snRNA. Another yeast Sm-like protein, Lsm1p, does not interact with U6 snRNA. Surprisingly, using the tandem affinity purification (TAP) method, we identified Lsm1p among the subunits associated with Lsm3p. Coprecipitation experiments demonstrated that Lsm1p, together with Lsm2p-Lsm7p, forms a new seven-subunit complex. We purified the two related Sm-like protein complexes and identified the proteins recovered in the purified preparations by mass spectrometry. This confirmed the association of the Lsm2p-Lsm8p complex with U6 snRNA. In contrast, the Lsm1p-Lsm7p complex is associated with Pat1p and Xrn1p exoribonuclease, suggesting a role in mRNA degradation. Deletions of LSM1, 6, 7 and PAT1 genes increased the half-life of reporter mRNAs. Interestingly, accumulating mRNAs were capped, suggesting a block in mRNA decay at the decapping step. These results indicate the involvement of a new conserved Sm-like protein complex and a new factor, Pat1p, in mRNA degradation and suggest a physical connection between decapping and exonuclease trimming.

A novel genetic screen for snRNP assembly factors in yeast identifies a conserved protein, Sad1p, also required for pre-mRNA splicing

The assembly pathway of spliceosomal snRNPs in yeast is poorly understood. We devised a screen to identify mutations blocking the assembly of newly synthesized U4 snRNA into a functional snRNP. Fifteen mutant strains failing either to accumulate the newly synthesized U4 snRNA or to assemble a U4/U6 particle were identified and categorized into 13 complementation groups. Thirteen previously identified splicing-defective prp mutants were also assayed for U4 snRNP assembly defects. Mutations in the U4/U6 snRNP components Prp3p, Prp4p, and Prp24p led to disassembly of the U4/U6 snRNP particle and degradation of the U6 snRNA, while prp17-1 and prp19-1 strains accumulated free U4 and U6 snRNA. A detailed analysis of a newly identified mutant, the sad1-1 mutant, is presented. In addition to having the snRNP assembly defect, the sad1-1 mutant is severely impaired in splicing at the restrictive temperature: the RP29 pre-mRNA strongly accumulates and splicing-dependent production of beta-galactosidase from reporter constructs is abolished, while extracts prepared from sad1-1 strains fail to splice pre-mRNA substrates in vitro. The sad1-1 mutant is the only splicing-defective mutant analyzed whose mutation preferentially affects assembly of newly synthesized U4 snRNA into the U4/U6 particle. SAD1 encodes a novel protein of 52 kDa which is essential for cell viability. Sad1p localizes to the nucleus and is not stably associated with any of the U snRNAs. Sad1p contains a putative zinc finger and is phylogenetically highly conserved, with homologues identified in human, Caenorhabditis elegans, Arabidospis, and Drosophila.

Association of snRNA genes with coiled bodies is mediated by nascent snRNA transcripts

BACKGROUND

Coiled bodies are nuclear organelles that are highly enriched in small nuclear ribonucleoproteins (snRNPs) and certain basal transcription factors. Surprisingly, coiled bodies not only contain mature U snRNPs but also associate with specific chromosomal loci, including gene clusters that encode U snRNAs and histone messenger RNAs. The mechanism(s) by which coiled bodies associate with these genes is completely unknown.

RESULTS

Using stable cell lines, we show that artificial tandem arrays of human U1 and U2 snRNA genes colocalize with coiled bodies and that the frequency of the colocalization depends directly on the transcriptional activity of the array. Association of the genes with coiled bodies was abolished when the artificial U2 arrays contained promoter mutations that prevent transcription or when RNA polymerase II transcription was globally inhibited by alpha-amanitin. Remarkably, the association was also abolished when the U2 snRNA coding regions were replaced by heterologous sequences.

CONCLUSIONS

The requirement for the U2 snRNA coding region indicates that association of snRNA genes with coiled bodies is mediated by the nascent U2 RNA itself, not by DNA or DNA-bound proteins. Our data provide the first evidence that association of genes with a nuclear organelle can be directed by an RNA and suggest an autogenous feedback regulation model.

Pop3p is essential for the activity of the RNase MRP and RNase P ribonucleoproteins in vivo

RNase MRP is a ribonucleoprotein (RNP) particle which is involved in the processing of pre-rRNA at site A3 in internal transcribed spacer 1. Although RNase MRP has been analysed functionally, the structure and composition of the particle are not well characterized. A genetic screen for mutants which are synthetically lethal (sl) with a temperature-sensitive (ts) mutation in the RNA component of RNase MRP (rrp2-1) identified an essential gene, POP3, which encodes a basic protein of 22.6 kDa predicted molecular weight. Over-expression of Pop3p fully suppresses the ts growth phenotype of the rrp2-1 allele at 34 degrees C and gives partial suppression at 37 degrees C. Depletion of Pop3p in vivo results in a phenotype characteristic of the loss of RNase MRP activity; A3 cleavage is inhibited, leading to under-accumulation of the short form of the 5.8S rRNA (5.8S(S)) and formation of an aberrant 5.8S rRNA precursor which is 5'-extended to site A2. Pop3p depletion also inhibits pre-tRNA processing; tRNA primary transcripts accumulate, as well as spliced but 5'- and 3'-unprocessed pre-tRNAs. The Pop3p depletion phenotype resembles those previously described for mutations in components of RNase MRP and RNase P (rrp2-1, rpr1-1 and pop1-1). Immunoprecipitation of epitope-tagged Pop3p co-precipitates the RNA components of both RNase MRP and RNase P. Pop3p is, therefore, a common component of both RNPs and is required for their enzymatic functions in vivo. The ubiquitous RNase P RNP, which has a single protein component in Bacteria and Archaea, requires at least two protein subunits for its function in eukaryotic cells.

Molecular cloning and subcellular localisation of the snRNP-associated protein 69KD, a structural homologue of the proto-oncoproteins TLS and EWS with RNA and DNA-binding properties

We recently isolated and characterised a 69 kDa protein (69KD) found associated with spliceosomal small nuclear ribonucleoproteins (snRNPs). Here, we report the molecular cloning of a cDNA encoding this protein, its nucleic acid binding properties and its subcellular localisation. Sequence analysis of the 69KD cDNA revealed: (1) that 69KD shares structural similarity with the human RNA binding proteins TLS and EWS (95% and 65% identity, respectively), the products of two genes frequently targeted by tumour-specific chromosomal translocations; (2) that 69KD contains a consensus RNA binding domain (CS-RBD) and three Arg/Gly-rich RNA binding motifs, structural features typical of many RNA binding proteins, in particular of hnRNP proteins; and (3) that 69KD contains a single putative Cys2/Cys2 zinc finger domain, a characteristic of many DNA-binding proteins. Consistent with its possession of these motifs, 69KD display a general nucleic acid binding activity, with a strong preference for guanyl and uridyl-rich RNA sequences, as well as for single-stranded and double-stranded DNA. The functional significance of this affinity for nucleic acids remains unclear. However, based on the established association of 69KD with the Sm core domain of snRNPs in vivo, these motifs might help mediate 69KD binding to snRNPs or be involved in some, as yet, unknown aspect of RNA metabolism. Consistent with both possibilities, 69KD is detected within typical snRNP containing subnuclear structures referred to as speckles, and is also more widely distributed throughout the nucleoplasm, as observed for many hnRNP proteins.

In vitro and in vivo evidence that protein and U1 snRNP nuclear import in somatic cells differ in their requirement for GTP-hydrolysis, Ran/TC4 and RCC1

GTP-hydrolysis, the small ras-related GTP-binding protein Ran and its cognate guanosine nucleotide exchange factor, the RCC1 gene product, have recently been identified as essential components of the protein nuclear import pathway. In this report we use three independent approaches to investigate the role of these components in U1 snRNP nuclear import in somatic cells. (i) Using a somatic cell based in vitro nuclear import system we show that U1 snRNP nuclear import, in marked contrast to protein transport, is not significantly inhibited by non-hydrolyzable GTP-analogs and is therefore unlikely to require GTP-hydrolysis. (ii) Using the dominant negative Ran mutant RanQ69L, which is defective in GTP-hydrolysis, we show that Ran-mediated GTP-hydrolysis is not essential for the nuclear import of U1 snRNP in microinjected cultured cells. (iii) Using a cell line expressing a thermolabile RCC1 gene product, we show that the nuclear accumulation of microinjected U1 snRNP is not significantly affected by RCC1 depletion at the non-permissive temperature, indicating that RCC1 function is not essential for U-snRNP nuclear import. Based on these observations we conclude that protein and U-snRNP nuclear import in somatic cells differ in their requirements for GTP-hydrolysis, and Ran or RCC1 function. Based on these results, the substrates for nucleocytoplasmic exchange across the NPC can be divided into two classes, those absolutely requiring Ran, including protein import and mRNA export, and those for which Ran is not essential, including U-snRNP nuclear import, together with tRNA and U1 snRNA nuclear export.

The yeast BDF1 gene encodes a transcription factor involved in the expression of a broad class of genes including snRNAs

While screening for genes that affect the synthesis of yeast snRNPs, we identified a thermosensitive mutant that abolishes the production of a reporter snRNA at the non-permissive temperature. This mutant defines a new gene, named BDF1. In a bdf1-1 strain, the reporter snRNA synthesized before the temperature shift remains stable at the non-permissive temperature. This demonstrates that the BDF1 gene affects the synthesis rather than the stability of the reporter snRNA and suggests that the BDF1 gene encodes a transcription factor. BDF1 is present in single copy on yeast chromosome XII, and is important for normal vegetative growth but not essential for cell viability. bdf1 null mutants share common phenotypes with several mutants affecting general transcription and are defective in snRNA production. BDF1 encodes a protein of 687 amino-acids containing two copies of the bromodomain, a motif also present in other transcription factors as well as a new conserved domain, the ET domain, also present in Drosophila and human proteins.

Differential interaction of splicing snRNPs with coiled bodies and interchromatin granules during mitosis and assembly of daughter cell nuclei

In the interphase nucleus of mammalian cells the U1, U2, U4/U6, and U5 small nuclear ribonucleoproteins (snRNPs), which are subunits of spliceosomes, associate with specific subnuclear domains including interchromatin granules and coiled bodies. Here, we analyze the association of splicing snRNPs with these structures during mitosis and reassembly of daughter nuclei. At the onset of mitosis snRNPs are predominantly diffuse in the cytoplasm, although a subset remain associated with remnants of coiled bodies and clusters of mitotic interchromatin granules, respectively. The number and size of mitotic coiled bodies remain approximately unchanged from metaphase to early telophase while snRNP-containing clusters of mitotic interchromatin granules increase in size and number as cells progress from anaphase to telophase. During telophase snRNPs are transported into daughter nuclei while the clusters of mitotic interchromatin granules remain in the cytoplasm. The timing of nuclear import of splicing snRNPs closely correlates with the onset of transcriptional activity in daughter nuclei. When transcription restarts in telophase cells snRNPs have a diffuse nucleoplasmic distribution. As cells progress to G1 snRNP-containing clusters of interchromatin granules reappear in the nucleus. Coiled bodies appear later in G1, although the coiled body antigen, p80 coilin, enters early into telophase nuclei. After inhibition of transcription we still observe nuclear import of snRNPs and the subsequent appearance of snRNP-containing clusters of interchromatin granules, but not coiled body formation. These data demonstrate that snRNP associations with coiled bodies and interchromatin granules are differentially regulated during the cell division cycle and suggest that these structures play distinct roles connected with snRNP structure, transport, and/or function.

The POP1 gene encodes a protein component common to the RNase MRP and RNase P ribonucleoproteins

Two forms of the yeast 5.8S rRNA are generated from a large precursor by distinct processing pathways. Cleavage at site A3 is required for synthesis of the major, short form, designated 5.8S(S), but not for synthesis of the long form, 5.8S(L). To identify components required for A3 cleavage, a bank of temperature-sensitive lethal mutants was screened for those with a reduced ratio of 5.8S(S):5.8S(L). The pop1-1 mutation (for processing of precursor RNAs) shows this phenotype and also inhibits A3 cleavage. The pre-rRNA processing defect of pop1-1 strains is similar to that reported for mutations in the RNA component of RNase MRP; we show that a mutation in the RNase MRP RNA also inhibits cleavage at site A3. This is the first site shown to require RNase MRP for cleavage in vivo. The pop1-1 mutation also leads to a block in the processing of pre-tRNA that is identical to that reported for mutations in the RNA component of RNase P. The RNA components of both RNase MRP and RNase P are underaccumulated in pop1-1 strains at the nonpermissive temperature, and immunoprecipitation demonstrates that POP1p is a component of both ribonucleoproteins. The POP1 gene encodes a protein with a predicted molecular mass of 100.5 kD and is essential for viability. POP1p is the first protein component of the nuclear RNase P or RNase MRP for which the gene has been cloned.