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PubMed1996

Proteins C1 and C2 of heterogeneous nuclear ribonucleoprotein complexes bind RNA in a highly cooperative fashion: support for their contiguous deposition on pre-mRNA during transcription.

Proteins C1 and C2 together comprise about one-third the protein mass of mammalian core 40S heterogeneous nuclear ribonucleoprotein particles (40S hnRNP) and exist as heterotetramers of (C1)3C2. On the basis of nonequilibrium binding studies, it has been suggested that the C proteins specifically bind oligo(U)- and poly(U)-rich sequences, and preferentially associate with uridine-rich regions near the 3' termini of many introns. We describe here a more quantitative characterization of the equilibrium binding properties of native and recombinant C protein to homoribopolymers using fluorescence spectroscopy. Like C protein from HeLa cells, the recombinant proteins spontaneously oligomerize to form tetramers with the same hydrodynamic properties as native protein. Near-stoichiometric binding titrations of the fluorescent homoribopolymer polyethenoadenosine (poly[r(epsilon A)]) with recombinant (C1)4 and (C2)4 homotetramers along with competition binding assays with poly(A) and poly(C) indicate that the binding site size (n) is between 150 and 230 nucleotides. This site size range is in close agreement with that previously determined for native C protein through hydrodynamic and ultrastructural studies (approximately 230 nucleotides). (C1)4 and (C2)4 bind poly(G) with intrinsic affinities (Ki) of 10(9) M-1, which are a hundredfold higher than their affinities for poly(U). In opposition to reports that C protein does not bind poly(A) and poly(C), we find that the C proteins bind these substrates with moderate Ki, but with high cooperativity (omega). The overall affinity (K omega) for the binding of both proteins to poly(A) and poly(C) is 10-fold higher (> 10(8) but < 10(9) M-1) than their affinities for poly(U). The highly cooperative binding of C protein to these substrates provides a mechanistic basis for the distribution of C protein along the length of nucleic acid substrates.

Binding Sites↗
PubMed2004

Proteomic characterization of messenger ribonucleoprotein complexes bound to nontranslated or translated poly(A) mRNAs in the rat cerebral cortex.

Receptor-triggered control of local postsynaptic protein synthesis plays a crucial role for enabling long lasting changes in synaptic functions, but signaling pathways that link receptor stimulation with translational control remain poorly known. Among the putative regulatory factors are mRNA-binding proteins (messenger ribonucleoprotein, mRNP), which control the fate of cytosolic localized mRNAs. Based on the assumption that a subset of mRNA is maintained in an inactive state, mRNP-mRNA complexes were separated into polysome-bound (translated) and polysome-free (nontranslated) fractions by sucrose density centrifugation. Poly(A) mRNA-mRNP complexes were purified from a postmitochondrial extract of rat cerebral cortex by oligo(dT)-cellulose affinity chromatography. The mRNA processing proteins were characterized, from solution, by a nanoflow reverse phase-high pressure liquid chromatography-mu-electrospray ionization mass spectrometry. The majority of detected mRNA-binding proteins was found in both fractions. However, a small number of proteins appeared to be fraction-specific. This subset of proteins is by far the most interesting because the proteins are potentially involved in controlling an activity-dependent onset of translation. They include transducer proteins, kinases, and anchor proteins. This study of the mRNP proteome is the first step in allowing future experimentation to characterize individual proteins responsible for mRNA processing and translation in dendrites.

Animals↗
PubMed1988

Splice site selection and ribonucleoprotein complex assembly during in vitro pre-mRNA splicing.

To study the determinants of splice site selection, we have inserted synthetic 5' and 3' splice sites at different positions within beta-globin genes and analyzed the resultant RNA substrates for in vitro splicing, factor binding, and complex assembly. We show that consensus 5' and 3' splice site sequences are insufficient to determine splice site utilization; in the presence or absence of the authentic site, the synthetic sites are variably active in a position-dependent manner. However, regardless of position or utilization, the synthetic 5' and 3' splice sites are bound by the appropriate splicing factors. Thus, binding of splicing factors is necessary but not sufficient for splice site utilization. Finally, we demonstrate that a block to efficient splicing can occur at multiple steps in the pathway of normal splicing complex assembly.

Animals↗
PubMed1988

Assembly of a polyadenylation-specific 25S ribonucleoprotein complex in vitro.

Extracts from HeLa cell nuclei assemble RNAs containing the adenovirus type 2 L3 polyadenylation site into a number of rapidly sedimenting heterodisperse complexes. Briefly treating reaction mixtures prior to sedimentation with heparin reveals a core 25S assembly formed with substrate RNA but not an inactive RNA containing a U----C mutation in the AAUAAA hexanucleotide sequence. The requirements for assembly of this heparin-stable core complex parallel those for cleavage and polyadenylation in vitro, including a functional hexanucleotide, ATP, and a uridylate-rich tract downstream of the cleavage site. The AAUAAA and a downstream U-rich element are resistant in the assembly to attack by RNase H. The poly(A) site between the two protected elements is accessible, but is attacked more slowly than in naked RNA, suggesting that a specific factor or secondary structure is located nearby. The presence of a factor bound to the AAUAAA in the complex is independently demonstrated by immunoprecipitation of a specific T1 oligonucleotide containing the element from the 25S fraction. Precipitation of this fragment from reaction mixtures is blocked by the U----C mutation. However, neither ATP nor the downstream sequence element is required for binding of this factor in the nuclear extract, suggesting that recognition of the AAUAAA is an initial event in complex assembly.

Adenoviridae↗
PubMed1998

Cellular protein hnRNP-A1 interacts with the 3'-end and the intergenic sequence of mouse hepatitis virus negative-strand RNA to form a ribonucleoprotein complex.

We previously showed that several cellular proteins specifically bind to the 3'-end and the intergenic sequences of the negative-strand RNA of mouse hepatitis virus (MHV), and proposed that these distant RNA sequences can be brought together by cellular and viral proteins (Furuya and Lai, 1993; Zhang et al., 1994; Zhang and Lai, 1995). The cellular protein p35 has been identified as a heterogeneous nuclear ribonucleoprotein (hnRNP) A1. We have now expressed hnRNP-A1 as a glutathione-S-transferase (GST) fusion protein and demonstrated that the amino terminal two-thirds of hnRNP-A1 interacted with the two MHV regulatory RNA sequences (3'-end and intergenic sequences) through protein-RNA interaction while its carboxy-terminal glycine-rich domain mediated homomeric (protein-protein) interactions. In a partially reconstituted reaction, in which the two MHV RNA fragments and the purified GST-hnRNP-A1 fusion protein were mixed, an RNP complex was formed. Depletion of either hnRNP-A1 or one of the RNA components abolished the complex formation. These results indicate that hnRNP-A1 can mediate the formation of an MHV RNP complex, which includes both the negative-strand leader and intergenic sequences. Site-directed mutagenesis revealed that mutations in the MHV intergenic sequences, which inhibited MHV RNA transcription, also inhibited the RNP complex formation. Deletion analysis showed that the amino terminal RNA-binding domains of hnRNP-A1 is essential for the RNP complex formation while the carboxy-terminal protein-binding domain enhanced the complex formation by 90-fold. These findings provide direct evidence demonstrating that the negative-strand leader RNA and intergenic sequences can form an RNP complex mediated by cellular protein hnRNP-A1.

Animals↗
PubMed1988

trans splicing in Leishmania enriettii and identification of ribonucleoprotein complexes containing the spliced leader and U2 equivalent RNAs.

The 5' ends of Leishmania mRNAs contain an identical 35-nucleotide sequence termed the spliced leader (SL) or 5' mini-exon. The SL sequence is at the 5' end of an 85-nucleotide primary transcript that contains a consensus eucaryotic 5' intron-exon splice junction immediately 3' to the SL. The SL is added to protein-coding genes immediately 3' to a consensus eucaryotic 3' intron-exon splice junction. Our previous work demonstrated possible intermediates in discontinuous mRNA processing that contain the 50 nucleotides of the SL primary transcript 3' to the SL, the SL intron sequence (SLIS). These RNAs have a 5' terminus at the splice junction of the SL and the SLIS. We examined a Leishmania nuclear extract for these RNAs in ribonucleoprotein (RNP) particles. Density centrifugation analysis showed that the SL RNA is predominantly in RNP complexes at 60S, while the SLIS-containing RNAs are in complexes at 40S. We also demonstrated that the SLIS can be released from polyadenylated RNA by incubation with a HeLa cell extract containing debranching enzymatic activity. These data suggested that Leishmania enriettii mRNAs are assembled by bimolecular or trans splicing as has been recently demonstrated for Trypanosoma brucei. Furthermore, we determined the partial sequence of the Leishmania U2 equivalent RNA and demonstrated that it cosediments with the SL RNA at 60S in a nuclear extract. These RNP particles may be analogous to so-called spliceosomes that have been demonstrated in other systems.

Animals↗
PubMed1983

Free messenger ribonucleoprotein complexes of chicken primary muscle cells following modification of protein synthesis by heat-shock treatment.

When primary cultures of chicken myoblasts were subjected to incubation at a temperature higher than their normal growing temperature of 36-37 degrees C, the pattern of protein synthesis was altered. This condition of heat shock induced a vigorous production of a number of proteins collectively known as 'heat-shock proteins'. The synthesis of heat-shock proteins was achieved without a significant decrease in the production of a broad spectrum of proteins by muscle cells. The synthesis of three major heat-shock polypeptides with Mr values of 81 000, 65 000 and 25 000 was observed in both mononucleated dividing myoblast cells and terminally differentiated myotubes. Two-dimensional electrophoretic separation of the heat-induced polypeptides synthesized by myogenetic cultures further established that same set of polypeptides with Mr of 65 000 (pI 6.0 and 5.5), 81 000 (pI 6.2) and 25 000 (pI 5.6 and 5.3) were produced in myoblasts and myotubes. The effect of the changes in pattern of protein synthesis on the mRNA and protein moieties of non-polysomal cytoplasmic mRNA-protein complexes (free mRNP) was examined. Free mRNP complexes sedimenting at 20-35 S were isolated from the post-ribosomal supernatant of both normal and heat-shocked myotube cultures by centrifugation in a sucrose gradient. A 10-20S RNA fraction isolated from these complexes stimulated protein synthesis in a cell-free system. The RNA fraction obtained from heat-shocked cells appeared to direct the synthesis of all three major heat-shock proteins. In contrast, synthesis of these polypeptides was not detected when RNA from free mRNP complexes of normal cells was used for translation. The free mRNP complexes of both normal and heat-shocked cells showed a buoyant density of 1.195 g/cm3 in metrizamide gradients. A large number of polypeptides of Mr = 35 000-105 000 were present in the highly purified free mRNP complexes isolated from the metrizamide gradient. Similar sets of polypeptides were found in these complexes from both normal and heat-shocked myotube culture. However, the relative proportion of a 65 000-Mr polypeptide was dramatically increased in the free mRNP complexes of heat-shocked cells. Two-dimensional gel electrophoretic analysis revealed that this polypeptide and the 65 000-Mr heat-shock polypeptide exhibit similar electrophoretic migration properties. These observations suggest that, following heat-shock treatment of chicken myotube cultures, the changes in the pattern of protein synthesis is accompanied by alteration of the mRNA and protein composition of free mRNP complexes.

Animals↗