Role In mRNA Metabolism Research Articles

RNA trafficking and stabilization elements associate with multiple brain proteins.

Two of the best understood somatic cell mRNA cytoplasmic trafficking elements are those governing localization of beta-actin and myelin basic protein mRNAs. These cis-acting elements bind the trans-acting factors fibroblast ZBP-1 and hnRNP A2, respectively. It is not known whether these elements fulfil other roles in mRNA metabolism. To address this question we have used Edman sequencing and western blotting to identify six rat brain proteins that bind the beta-actin element (zipcode). All are known RNA-binding proteins and differ from ZBP-1. Comparison with proteins that bind the hnRNP A2 and AU-rich response elements, A2RE/A2RE11 and AURE, showed that AURE and zipcode bind a similar set of proteins that does not overlap with those that bind A2RE11. The zipcode-binding protein, KSRP, and hnRNP A2 were selected for further study and were shown by confocal immunofluorescence microscopy to have similar distributions in the central nervous system, but they were found in largely separate locations in cell nuclei. In the cytoplasm of cultured oligodendrocytes they were segregated into separate populations of cytoplasmic granules. We conclude that not only may there be families of trans-acting factors for the same cis-acting element, which are presumably required at different stages of mRNA processing and metabolism, but independent factors may also target different and multiple RNAs in the same cell.

Expression of the RNA binding proteins, Mel-N1, Mel-N2, and Mel-N3 in adipose cells

Mel-N1 (murine embryonic lethal abnormal vision [ELAV]), a mammalian homolog of Drosophila ELAV, is an mRNA binding protein of the RNA Recognition Motif family. Studies with the human homolog, Hel-N1 have supported the hypothesis that Hel-N1, and its splice variant, Hel-N2 play a role in mRNA metabolism. Thus it becomes logical to extend this hypothesis to the murine variant Mel-N1 which has been described as a neuronal protein with a minor level of expression in the testis. Our current work expands the potential function for this protein through demonstration of expression of the full-length message and splice variants in adipose tissue as well as preadipocyte and adipocyte cell lines.

Schizosaccharomyces pombe spPABP, a homologue of Saccharomyces cerevisiae Pab1p, is a non-essential, shuttling protein that facilitates mRNA export.

Poly(A)-binding proteins play important roles in mRNA metabolism in eukaryotic cells. We examined the role of the Schizosaccharomyces pombe homologue of the Saccharomyces cerevisiae poly(A)-binding protein, Pab1p, in cellular growth and mRNA export. In contrast to PAB1, the sppabp gene is not essential for cellular viability. Like the human hPABP1 protein, spPABP is cytoplasmically localized and can shuttle between the nucleus and the cytoplasm. We found that a spPABP-GFP fusion protein expressed from a multicopy plasmid could suppress the growth and mRNA export defect of rae1-16 7 nup184-1 synthetic lethal mutations. However, about 20-25% of cells in the population exhibited a pronounced nuclear accumulation of poly(A)(+) RNA. The same cells also localized the spPABP-GFP fusion to the nucleus, suggesting that the shuttling ability of spPABP is related to its function in mRNA export. When a heterologous nuclear export activity from spMex67p was fused to spPABP-GFP fusion protein, it overcame the nuclear retention but did not increase nuclear mRNA export. We discuss the implications of these observations in relation to how spPABP could function in mRNA export. Published in 2002 by John Wiley & Sons, Ltd.

Transcription-Independent RNA Polymerase II Dephosphorylation by the FCP1 Carboxy-Terminal Domain Phosphatase in Xenopus laevis Early Embryos

The phosphorylation of the RNA polymerase II (RNAP II) carboxy-terminal domain (CTD) plays a key role in mRNA metabolism. The relative ratio of hyperphosphorylated RNAP II to hypophosphorylated RNAP II is determined by a dynamic equilibrium between CTD kinases and CTD phosphatase(s). The CTD is heavily phosphorylated in meiotic Xenopus laevis oocytes. In this report we show that the CTD undergoes fast and massive dephosphorylation upon fertilization. A cDNA was cloned and shown to code for a full-length xFCP1, the Xenopus orthologue of the FCP1 CTD phosphatases in humans and Saccharomyces cerevisiae. Two critical residues in the catalytic site were identified. CTD phosphatase activity was observed in extracts prepared from Xenopus eggs and cells and was shown to be entirely attributable to xFCP1. The CTD dephosphorylation triggered by fertilization was reproduced upon calcium activation of cytostatic factor-arrested egg extracts. Using immunodepleted extracts, we showed that this dephosphorylation is due to xFCP1. Although transcription does not occur at this stage, phosphorylation appears as a highly dynamic process involving the antagonist action of Xp42 mitogen-activated protein kinase and FCP1 phosphatase. This is the first report that free RNAP II is a substrate for FCP1 in vivo, independent from a transcription cycle.

Efficient Synthesis of 13C,15N-Labeled RNA Containing the Cap Structure m7GpppA

Studying the mechanism by which the cap structure performs its numerous biological roles in mRNA metabolism by NMR spectroscopy requires the large-scale production of isotopically labeled capped RNA. We present an efficient method for production of short RNA containing the cap structure m7GpppA, by combining chemical synthesis with enzymatic synthesis using a DNA primase. This method was employed to synthesize three capped RNA molecules, m7Gppp[13C,15N]A, m7Gppp[13C,15N]ACC, and m7GpppA[13C,15N]C[13C,15N]C, that contain selective 13C,15N-labeled adenosine or cytosine nucleotides. This selective labeling technique enabled us to obtain the chemical shift assignments of these oligonucleotides in complex with the eukaryotic translation initiation factor 4E (eIF4E). Furthermore, we were able to observed intermolecular NOE interactions between the RNA and protein.

SUG1, a Component of the 26 S Proteasome, Is an ATPase Stimulated by Specific RNAs

SUG1 is an integral component of the 26 S proteasome. Belonging to a novel putative ATPase family, it shares four conserved motifs characteristic of ATP-dependent DNA/RNA helicases. Recombinant rat SUG1 (rSUG1) produced in Escherichia coli was highly purified and characterized in terms of its biochemical properties. The rSUG1 exhibited a Mg2+-dependent ATPase activity. The Km for ATP and Vmax of rSUG1 were 35 microM and 7 pmol of ATP/min/microg of protein, respectively. Both ATPase activity to release [32P]monophosphate and [32P]ATP-labeling activity were coordinately affected by cold ATP severely, GTP and UTP moderately, and CTP little. Interestingly, the rSUG1 ATPase activity was stimulated by poly(U) and poly(C), but not by poly(A), poly(G), or by any forms of DNAs tested. A UV cross-linking assay also indicated poly(U)- and poly(C)-stimulated labeling of rSUG1 with [alpha-32P]ATP. Moreover, the ATPase activity was facilitated by cellular poly(A)+ RNA, but not by poly(A)- RNA. RNA transcribed in vitro from cDNA encoding a b-Zip protein could stimulate the ATPase activity. This is the first report to demonstrate a specific RNA requirement for ATPase with respect to the proteasomal ATPases. Our present work suggests that SUG1 can specifically interact with protein-coding RNA (mRNA) and play some roles in mRNA metabolism.

Missense mutation clustering in the survival motor neuron gene: a role for a conserved tyrosine and glycine rich region of the protein in RNA metabolism?

The Survival Motor Neuron (SMN) gene shows deletions in the majority of patients with Spinal Muscular Atrophy (SMA), a disease of motor neuron degeneration. To date only two missense mutations have been reported in SMN in patients with SMA. The fact that no SMN-homologues have been forthcoming from data-base searching has resulted in a lack of hypotheses concerning the structural and functional consequences of these mutations. Recently SMN has been shown to interact with heterogeneous nuclear ribonucleoproteins (hnRNPs) suggesting a role in mRNA metabolism. We describe a novel missense mutation and the subsequent identification of a triplicated tyrosine-glycine (Y-G) peptide sequence at the C-terminal of SMN which encompasses each of the three predicted amino acid sequence substitutions. We have identified apparent orthologues of SMN in Caenorhabditis elegans and Schizosaccharomyces pombe. These sequences retain the highly conserved Y-G motif and provide additional support for a role of SMN in mRNA metabolism.

The Drosophila RNA-binding protein RBP1 is localized to transcriptionally active sites of chromosomes and shows a functional similarity to human splicing factor ASF/SF2.

An RNA-binding protein gene (rbp1) from Drosophila melanogaster, encoding an RNA recognition motif and an Arg-Ser rich (RS) domain, has been characterized. The predicted amino acid sequence of rbp1 is similar to those of the human splicing factor ASF/SF2, the Drosophila nuclear phosphoprotein SRp55, and the Drosophila puff-associated protein B52. Northern and immunohistochemical analyses showed that rbp1 is expressed at all stages in all tissues and that the RBP1 protein is localized to the nucleus. Consistent with a role in mRNA metabolism, indirect immunofluorescence reveals that the RBP1 protein colocalizes with RNA polymerase II on larval salivary gland polytene chromosomes. RBP1 protein made in Escherichia coli was tested for splicing activity using human cell extracts in which ASF has been shown previously both to activate splicing and to affect the choice of splice sites in alternatively spliced pre-mRNAs. In these assays, RBP1 protein, like ASF, is capable of both activating splicing and switching splice site selection. However, in each case, clear differences in the behavior of the two proteins were detected, suggesting that they have related but not identical functions. The general nuclear expression pattern, colocalization on chromosomes with RNA polymerase II, the similarity to ASF/SF2, SRp55, and B52, along with the effect on alternative splicing shown in vitro, suggest that rbp1 is involved in the processing of precursor mRNAs.

Modulation of eucaryotic initiation factor-4E binding to 5'-capped oligoribonucleotides by modified anti-sense oligonucleotides.

The 5' cap structure of eucaryotic mRNA plays a pivotal role in mRNA metabolism. This report demonstrates that anti-sense oligonucleotides equipped with 3'-overhanging nucleotides modulate the amount of recombinant human eucaryotic initiation factor-4E that binds to a 5'-capped oligoribonucleotide. The degree of inhibition or enhancement of protein binding is dependent upon the number and sequence of overhanging nucleotides. A 45% inhibition of complexation was observed by the addition of one 3'-overhanging guanosine residue. Addition of a second residue (+2/GN) resulted in a higher degree of inhibition, 77-88%. In contrast, addition of one adenosine residue enhanced the formation of the eucaryotic initiation factor-4E-m7GpppRNA complex by 213%. Modulation of protein interactions with the 5'-cap structure is likely to effect several biological events, including pre-mRNA processing, transport of the mRNA from the nucleus to the cytoplasm and translation of the target mRNA. This targeting strategy in anti-sense chemistry may have practical applications in experimental biology and medicine.

Growth factor regulated expression of poly(A) + binding protein messenger RNA

A 72,000 mol wt protein designated PABP binds to the poly(A) + track of messenger RNAs with high affinity and has been suggested to play an important role in mRNA metabolism in eucaryotic cells. We have employed a human PABP cDNA probe to study the expression of this gene at the mRNA level in BALB/c3T3 mouse cells under different growth conditions and in exponentially growing HeLa cells throughout the cell division cycle. We describe experiments which establish that in BALB/c3T3 cells the expression of this gene is growth factor regulated. Moreover, the gene behaves like a primary response gene in that its induction in quiescent cells does not require the prior synthesis of other growth factor-regulated proteins. In exponentially growing HeLa cells PABP mRNA is expressed throughout the cell division cycle indicating that the expression of this gene is not limited to a specific phase of the cell cycle.