Adenosine-uridine Binding Factor Research Articles

Canine brain glucose transporter 3: Gene sequence, phylogenetic comparisons and analysis of functional sites

There has been a sparsity of various mammalian neuronal glucose transporter 3-encoding sequences ( Glut3) available for the purposes of alignment studies. We report here a 2355-bp sequence of canine Glut3 that encodes a deduced protein of 496 amino acids (aa). The full-length canine aa sequence was compared to those of the human, mouse and rat glucose transporter 3 (Glut3), and found to be 88.3, 84.9 and 84.3% identical, respectively. However, while mouse and rat have identical C-termini, the canine and human C-termini share markedly little identity or similarity to one another, or to that of rat/mouse. The canine Glut3 sequence also exhibits 74.5% aa identity with a non-mammalian chicken Glut3 sequence. These differences in the C-termini of Glut3 among the species may result in kinetic or mechanistic differences in the transport of glucose. Computer searches were made for conserved functional motifs, and a brief review of ten sites is provided. This review includes the determination of their locations in two transmembrane (TM) motifs that have been proposed for glucose transporters. The nucleotide (nt) sequence of the 5′-untranslated region ( UTR) of canine Glut3 was aligned with the comparable human Glut3 region and was shown to be 70% identical over a region of 129 nt just prior to the ATG start codons. A similar comparison of the 3′-UTR shows 74% identity over 350 nt immediately following the stop codons. An adenosine-uridine-binding factor (AUBF) region, which has been identified as a region of importance in mRNA stabilization, is conserved in the 3′-UTR of both canine and human Glut3. The conservation in the UTR suggests that Glut3 may be post-transcriptionally regulated.

Bovine chondrocyte link protein cDNA sequence: interspecies conservation of primary structure and mRNA untranslated regions

The sequence for bovine link protein cDNA, including 108 bases of the 5′ untranslated region (UTR) and 768 nucleotides of the 3′ UTR, was determined from polymerase chain reaction products and bovine articular chondrocyte cDNA clones. The deduced primary structure for bovine link protein predicts a protein 354 amino acid residues in length. Comparative analysis with link protein sequence from several other species revealed overall high conservation of protein coding sequence. High nucleotide sequence conservation was observed within the extensive 5′ and 3′ UTRs of bovine, human, pig, chick and rat link protein mRNA. As evidence that the UTRs might play a role in regulation of link protein mRNA turnover, multiple occurrences of the adenosine-uridine binding factor motif A(Un)A were found to be conserved between species within 3' UTRs. A polyadenylation signal was conserved between the bovine and chicken sequence, use of which would result in the smallest of multiple bovine link protein mRNA species observed by Northern blot analysis.

Modulation of granulocyte-macrophage colony-stimulating factor mRNA stability in vitro by the adenosine-uridine binding factor.

In vitro decay of granulocyte-macrophage colony-stimulating factor (GM-CSF) mRNA was examined on polysomes prepared from normal human peripheral blood mononuclear cells stimulated with phorbol ester (TPA) and phytohemagglutinin for 14 h. GM-CSF mRNA decayed with a half-life of 90 min while 18 S rRNA was stable. RNA gel mobility assay performed on crude cytosolic lysate (S20) with radiolabeled AUUUA containing RNA identified a 42-kDa RNA-protein complex on SDS-polyacrylamide gel electrophoresis. The binding specificity was identical to that of the previously described adenosine-uridine binding factor (AUBF) (Malter, J. S. (1989) Science 246, 664-666). Further fractionation of the S20 cytosol through a sucrose gradient showed > 90% of AUBF activity associated with polysomes and < 10% with the S130 fraction. Solution phase RNAs containing AUUUA reiterations specifically competed for polysome-bound AUBF and accelerated the decay of GM-CSF mRNA (t1/2 = 17 min). We linked biotinylated AUUUA RNA to streptavidin magnetic beads and removed > 95% of polysome-associated AUBF. A decay system thus depleted of AUBF activity also showed accelerated decay of GM-CSF mRNA (t1/2 = 20 min). These data show that AUBF is preferentially located on polysomes and that its removal destabilizes GM-CSF mRNA. Therefore, AUBF likely prevents GM-CSF mRNA decay by binding to the AUUUA instability determinants in the 3'-untranslated region.

Association of AUUUA-binding Protein with A + U-rich mRNA during nucleo-cytoplasmic transport

Resealed nuclear envelope (NE) vesicles from rat liver containing entrapped exogenous RNA were used to study the effect of adenosine + uridine binding factor (AUBF), present in cytosolic cell extracts, on ATP-dependent transport of A + U-rich RNA (AU +RNA) and A + U-free RNA (AU −RNA) across the NE. This factor specifically binds to A + U-rich sequences present in the 3′ untranslated regions of lymphokine and cytokine mRNAs. containing overlapping AUUUA boxes (granulocyte—macrophage colony stimulating factor interleukin-3). Addition of AUBF to the extravesicular compartment markedly increased the efflux of the in vitro transcribed, capped and polyadenylated AU + RNAs. Export of entrapped AU − control RNA, such as β-globin RNA, was not affected by AUBF, in contrast to chimeric AU + β-globin RNA containing the A + U-rich sequence of human Interferon-α mRNA (6 reiterated AUUUA motifs). Competition experiments revealed that AUBF enhances the affinity of poly(A)-containing AU + RNAs to the NE poly(A)-binding component (poly(A)-recognizing mRNA carrier p106), and thereby accelerates nuclear export of these RNAs. We could demonstrate that AUBF added to the extravesicular space forms stable complexes with polyadenylated AU + RNA with relative molecular masses of about 45,000, 62,000 and 70,000 inside the vesicles or during ATP-dependent export. In addition we determined that AUBF may affect mRNA stability by protecting A + U-rich RNA against degradation by trans-acting, nuclear matrix-associated and A + U-specific endoribonuclease V.

Tumor necrosis factor alpha-induced glucose transporter (GLUT-1) mRNA stabilization in 3T3-L1 preadipocytes. Regulation by the adenosine-uridine binding factor.

Tumor necrosis factor alpha (TNF alpha), 12-O-tetradecanoylphorbol-13-acetate and cAMP stimulate hexose transport in quiescent 3T3-L1 preadipocytes by stabilizing the relatively labile mRNA coding for the basal glucose transporter, GLUT-1. The 3'-UTR of GLUT-1 mRNA contains a single copy of the destabilizing AUUUA motif in the context of an AU-rich region. The adenosine-uridine binding factor (AUBF) is a cytosolic protein which interacts with similar AU-rich regions in a variety of labile cytokine and oncogene mRNAs. Here, we demonstrate that AUBF complexes in vitro with GLUT-1 mRNA through the AU-rich portion of the 3'-UTR. AUBF activity is very low in quiescent preadipocytes, but can be up-regulated by agonists such as TPA, TNF alpha, cAMP, and okadaic acid, all of which stabilize GLUT-1 mRNA. The time courses of TNF alpha- and TPA-mediated AUBF up-regulation and GLUT-1 mRNA stabilization are coincident, suggesting a cause and effect relationship.

A redox switch and phosphorylation are involved in the post-translational up-regulation of the adenosine-uridine binding factor by phorbol ester and ionophore.

Messenger RNAs coding for cytokines and lymphokines are extremely unstable due to an AU-rich cis element located in their 3'-untranslated region. Cell activation with phorbol ester 12-O-tetradecanoylphorbol-13-acetate or calcium ionophore has been shown to markedly stabilize these normally labile messages. We have recently described a cytoplasmic protein, denoted the adenosine-uridine binding factor (AUBF) which complexes in vitro to a variety of labile RNAs containing multiple reiterations of the pentamer AUUUA. In order to determine if AUBF plays a role in the stabilization of cytokine and lymphokine mRNAs, we have investigated the mechanisms which control AUBF activity in peripheral blood mononuclear cells as well as Jurkat cells. AUBF is inactive in resting peripheral blood mononuclear cells but can be activated by brief treatment with 12-O-tetradecanoylphorbol-13-acetate or ionophore. Up-regulation is independent of protein synthesis or RNA transcription, suggesting pre-existing AUBF is subject to post-translational modification. AUBF activity can be reversibly blocked by diamide but irreversibly inhibited by n-ethylmaleimide, suggesting that AUBF contains a redox switch as described for other RNA-binding proteins. Finally, AUBF activity is abolished by potato acid phosphatase, demonstrating that AUBF is a phosphoprotein. These data demonstrate that AUBF activity is subject to at least two levels of post-translational regulation and is enhanced by mitogens previously shown to induce the stabilization of AUUUA mRNAs. Based upon these data, we propose that AUBF binding may mediate 12-O-tetradecanoylphorbol-13-acetate and ionophore-mediated labile message stabilization.

The adenosine-uridine binding factor recognizes the AU-rich elements of cytokine, lymphokine, and oncogene mRNAs.

Selective mRNA degradation is an important control point in the transient expression of a variety of mRNAs coding for growth regulators. A variety of labile mRNAs coding for lymphokines, cytokines, and oncogenes contain within their 3'-untranslated region an AU-rich region shown to destabilize these messages. We recently identified a cytosolic protein, adenosine-uridine binding factor (AUBF), which complexes with four tandem AUUUA reiterations of a synthetic RNA transcript. We now show that AUBF forms RNase T1-resistant band-shifted complexes with a variety of in vitro transcribed mRNAs including granulocyte-macrophage colony-stimulating factor, interferon-gamma, interleukin-3, c-fos, and v-myc. Formation of complexes was specifically inhibited by AUUUA containing RNA, but not by irrelevant RNA. After brief ultraviolet light-induced cross-linking, AUBF.RNA complexes with the exception of c-fos comigrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Mutations within the AUUUA motifs demonstrate that both nucleotide sequence and secondary structure are important in AUBF.AUUUA RNA complex formation. Based upon these data, we suggest AUBF may interact with a variety of labile mRNAs with multiple AUUUA reiterations or single reiterations within an AU-rich 3'-untranslated region.

Adenosine-Uridine Binding Factor Requires Metals for Binding to Granulocyte-Macrophage Colony-Stimulating Factor mRNA

Post-transcriptional gene regulation plays an important role in the expression of granulocyte-macrophage colony-stimulating factor (GM-CSF). Cytokine secretion by activated lymphocytes or mast cells is preceded by dramatic stabilization of the normally labile GM-CSF mRNA. The 3'-untranslated region of GM-CSF and other labile mRNAs contain the destabilizing motif adenosine-uridine-uridine-uridine-adenosine (AUUUA). We recently identified a cytoplasmic protein denoted the adenosine-uridine binding factor (AUBF) which binds with high affinity and specificity to AUUUA elements in synthetic RNA transcripts. We now demonstrate that AUBF binds specifically to GM-CSF mRNA through the destabilizing AUUUA elements. The formation of AUBF-GM-CSF RNA complexes required calcium or magnesium which were sensitive to EDTA or EGTA. A variety of other divalent metals blocked magnesium-dependent AUBF activity. These observations suggest that AUBF may protect GM-CSF mRNA from rapid degradation and play a crucial role in the expression of cytokine genes.