Multiple functional interactions between components of the Lsm2-Lsm8 complex, U6 snRNA, and the yeast La protein

Ty3 requires yeast La homologous protein for wild-type frequencies of transposition

The Saccharomyces cerevisiae retrovirus-like element Ty3 inserts specifically into the initiation sites of genes transcribed by RNA polymerase III (pol III). A strain with a disruption of LHP1, which encodes the homologue of autoantigen La protein, was recovered in a screen for mutants defective in Ty3 transposition. Transposition into a target composed of divergent tRNA genes was decreased eightfold. In lhp1 mutants, Ty3 polyproteins were produced at wild-type levels, assembled into virus-like particles (VLPs) and processed efficiently. The amount of cDNA associated with these particles was about half the amount in a wild-type control at early times, but approached the wild-type level after 48 h of induction. Ty3 integration was examined at two genomic tRNA gene families and two plasmid-borne tRNA promoters. Integration was significantly decreased at one of the tRNA gene families, but was only slightly decreased at the second tRNA gene family. These findings suggest that Lhp1p contributes to Ty3 cDNA synthesis, but might also act at a target-specific step, such as integration.

The human LSm1-7 proteins colocalize with the mRNA-degrading enzymes Dcp1/2 and Xrnl in distinct cytoplasmic foci

Sm and Sm-like (LSm) proteins form heptameric complexes that are involved in various steps of RNA metabolism. In yeast, the Lsm1-7 complex functions in mRNA degradation and is associated with several enzymes of this pathway, while the complex LSm2-8, the composition of which largely overlaps with that of LSm1-7, has a role in pre-mRNA splicing. A human gene encoding an LSm1 homolog has been identified, but its role in mRNA degradation has yet to be elucidated. We performed subcellular localization studies and found hLSm1 predominantly in the cytoplasm. However, it is not distributed evenly; rather, it is highly enriched in small, discrete foci. The endogenous hLSm4 is similarly localized, as are the overexpressed proteins hLSm1-7, but not hLSm8. The foci also contain two key factors in mRNA degradation, namely the decapping enzyme hDcp1/2 and the exonuclease hXrn1. Moreover, coexpression of wild-type and mutant LSm proteins, as well as fluorescence resonance energy transfer (FRET) studies, indicate that the mammalian proteins hLSm1-7 form a complex similar to the one found in yeast, and that complex formation is required for enrichment of the proteins in the cytoplasmic foci. Therefore, the foci contain a partially or fully assembled machinery for the degradation of mRNA.

The 5' and 3' domains of yeast U6 snRNA: Lsm proteins facilitate binding of Prp24 protein to the U6 telestem region

The 5' and 3' domains of yeast U6 snRNA contain sequences that are thought to be important for binding to Prp24 and Lsm proteins. By extensive mutational analysis of yeast U6 snRNA, we confirmed that the 3' terminal uridine tract of U6 snRNA is important for U6 binding to Lsm proteins in yeast. Binding of Prp24 protein to U6 RNA is dependent on or is strongly enhanced by U6 binding of Lsm proteins. This supports a model for U6 snRNP assembly in which U6 RNA binds to the Lsm2-8 core prior to binding Prp24 protein. Using compensatory base-pairing analysis, we show that at least half of the recently identified U6 telestem as well as a nucleotide sequence in the other half of the telestem are important for binding of U6 RNA to Prp24 protein. Surprisingly, disruption of base pairing in the unconfirmed half of the telestem enhanced U6-Prp24 binding. Truncation of the entire 3' terminal domain or nearly the entire 5' terminal domain of yeast U6 allowed for detectable levels of splicing to proceed in vitro. In addition to gaining knowledge of the function of the 5' and 3' domains of yeast U6, our results help define the minimal set of requirements for yeast U6 RNA function in splicing. We present a revised secondary structural model of yeast U6 snRNA in free U6 snRNPs.

Structural Modelling of the Sm-like Protein Hfq from Escherichia coli

The Hfq polypeptide of Escherichia coli is a nucleic acid-binding protein involved in the expression of many proteins. Derivation of its three-dimensional structure is important for our understanding of its role in gene regulation at the molecular level. In this study, we combined computational and biophysical analysis to derive a possible structure for Hfq. As a first step towards determining the structure, we searched for possible sequence-structure compatibility, using secondary structure prediction and protein domain and fold-recognition methods available on the WEB. One fold, essentially beta sheet in character, the Sm motif of small nuclear ribonucleoproteins, even though it initially fell well below the confidence thresholds, was proposed and further validated by a series of biophysical and biochemical studies. The Hfq hexamer structure was modelled on the human Sm D3B structure using optimised sequence alignments and molecular mechanics methods. This structure accounts for the physico-chemical properties of Hfq and highlights amino acid residues that could interact with RNA.

Lsm proteins are required for normal processing of pre-tRNAs and their efficient association with La-homologous protein Lhp1p

Depletion of any of the five essential proteins Lsm2p to Lsm5p and Lsm8p leads to strong accumulation of all tested unspliced pre-tRNA species, as well as accumulation of 5' and 3' unprocessed species. Aberrant 3'-extended pre-tRNAs were detected, presumably due to stabilization of transcripts that fail to undergo correct transcription termination, and the accumulation of truncated tRNA fragments was also observed. Tandem affinity purification-tagged Lsm3p was associated with pre-tRNA primary transcripts and, less efficiently, with other unspliced pre-tRNA intermediates but not mature tRNAs. Association of the Saccharomyces cerevisiae La homologue Lhp1p with pre-tRNAs was reduced approximately threefold on depletion of Lsm3p or Lsm5p. The association of Lhp1p with larger RNA polymerase III transcripts, pre-RNase P RNA and the signal recognition particle RNA (scR1), was more drastically reduced. The impaired pre-tRNA processing seen on Lsm depletion is not, however, due solely to reduced Lhp1p association as evidenced by analysis of lhp1-Delta strains depleted of Lsm3p or Lsm5p. These data are consistent with roles for an Lsm complex as a chaperone that facilitates the efficient association of pre-tRNA processing factors with their substrates.

Interaction generality, a measurement to assess the reliability of a protein-protein interaction

Here we introduce the 'interaction generality' measure, a new method for computationally assessing the reliability of protein-protein interactions obtained in biological experiments. This measure is basically the number of proteins involved in a given interaction and also adopts the idea that interactions observed in a complicated interaction network are likely to be true positives. Using a group of yeast protein-protein interactions identified in various biological experiments, we show that interactions with low generalities are more likely to be reproducible in other independent assays. We constructed more reliable networks by eliminating interactions whose generalities were above a particular threshold. The rate of interactions with common cellular roles increased from 63% in the unadjusted estimates to 79% in the refined networks. As a result, the rate of cross-talk between proteins with different cellular roles decreased, enabling very clear predictions of the functions of some unknown proteins. The results suggest that the interaction generality measure will make interaction data more useful in all organisms and may yield insights into the biological roles of the proteins studied.

La protein and its associated small nuclear and nucleolar precursor RNAs

After transcription by RNA polymerase (pol) III, nascent Pol III transcripts pass through RNA processing, modification, and transport machineries as part of their posttranscriptional maturation process. The first factor to interact with Pol III transcripts is La protein, which binds principally via its conserved N-terminal domain (NTD), to the UUU-OH motif that results from transcription termination. This review includes a sequence Logo of the most conserved region of La and its refined modeling as an RNA recognition motif (RRM). La protects RNAs from 3' exonucleolytic digestion and also contributes to their nuclear retention. The variety of modifications found on La-associated RNAs is reviewed in detail and considered in the contexts of how La may bind the termini of structured RNAs without interfering with recognition by modification enzymes, and its ability to chaperone RNAs through multiple parts of their maturation pathways. The CTD of human La recognizes the 5' end region of nascent RNA in a manner that is sensitive to serine 366 phosphorylation. Although the CTD can control pre-tRNA cleavage by RNase P, a rate-limiting step in tRNASerUGA maturation, the extent to which it acts in the maturation pathway(s) of other transcripts is unknown but considered here. Evidence that a fraction of La resides in the nucleolus together with recent findings that several Pol III transcripts pass through the nucleolus is also reviewed. An imminent goal is to understand how the bipartite RNA binding, intracellular trafficking, and signal transduction activities of La are integrated with the maturation pathways of the various RNAs with which it associates.

Human ribonuclease P: subunits, function, and intranuclear localization

Catalytic complexes of nuclear ribonuclease P (RNase P) ribonucleoproteins are composed of several protein subunits that appear to have specific roles in enzyme function in tRNA processing. This review describes recent progress made in the characterization of human RNase P, its relationship with the ribosomal RNA processing ribonucleoprotein RNase MRP, and the unexpected evolutionary conservation of its subunits. A new model for the biosynthesis of human RNase P is presented, in which this process is dynamic, transcription-dependent, and implicates functionally distinct nuclear compartments in tRNA biogenesis.

The La protein

Ubiquitous in eukaryotic cells, the La protein associates with the 3' termini of many newly synthesized small RNAs. RNAs bound by the La protein include all nascent transcripts made by RNA polymerase III as well as certain small RNAs synthesized by other RNA polymerases. Recent genetic and biochemical analyses have revealed that binding by the La protein protects the 3' ends of these RNAs from exonucleases. This La-mediated stabilization is required for the normal pathway of pre-tRNA maturation, facilitates assembly of small RNAs into functional RNA-protein complexes, and contributes to nuclear retention of certain small RNAs. Studies of mutant La proteins have given some insights into how the La protein specifically recognizes its RNA targets. However, many questions remain regarding the molecular mechanisms by which La protein binding influences multiple steps in small RNA biogenesis. This review focuses on the roles of the La protein in small RNA biogenesis and also discusses data that implicate the La protein in the translation of specific mRNAs.

The yeast cytoplasmic LsmI/Pat1p complex protects mRNA 3' termini from partial degradation

A key aspect of understanding eukaryotic gene regulation will be the identification and analysis of proteins that bind mRNAs and control their function. Recently, a complex of seven Lsm proteins and the Pat1p have been shown to interact with yeast mRNAs and promote mRNA decapping. In this study we present several observations to indicate that the LsmI/Pat1 complex has a second distinct function in protecting the 3'-UTR of mRNAs from trimming. First, mutations in the LSM1 to LSM7, as well as PAT1, genes led to the accumulation of MFA2pG and PGK1pG transcripts that had been shortened by 10-20 nucleotides at their 3' ends (referred to as trimming). Second, the trimming of these mRNAs was more severe at the high temperature, correlating with the inability of these mutant strains to grow at high temperature. In contrast, trimming did not occur in a dcp1 Delta strain, wherein the decapping enzyme is lacking. This indicates that trimming is not simply a consequence of the inhibition of mRNA decapping. Third, the temperature-sensitive growth of lsm and pat1 mutants was suppressed by mutations in the exosome or the functionally related Ski proteins, which are required for efficient 3' to 5' mRNA degradation of mRNA. Moreover, in lsm ski double mutants, higher levels of the trimmed mRNAs accumulated, indicating that exosome function is not required for mRNA trimming but that the exosome does degrade the trimmed mRNAs. These results raise the possibility that the temperature-sensitive growth of the lsm1-7 and pat1 mutants is at least partially due to mRNA trimming, which either inactivates the function of the mRNAs or makes them available for premature 3' to 5' degradation by the exosome.