RESUMO
The alternative splicing of the last intron (intron D) of bovine growth hormone (bGH) pre-mRNA requires a down-stream exonic splicing enhancer (FP/ESE). The presence of at least one SR protein has been shown to be essential for FP/ESE function and splicing of intron D in in vitro splicing assays. However, in vitro reconstitution of splicing using individual purified SR proteins may not accurately reflect the true complexity of alternative splicing in an intact nucleus, where multiple SR proteins in varying amounts are likely to be available simultaneously. Here, a panel of recombinant baculovirus-expressed SR proteins was produced and tested for the ability to activate FP/ESE-dependent splicing. Individual recombinant SR proteins differed significantly in their activity in promoting intron D splicing. Among the recombinant SR proteins tested, SRp55 was the most active, SC35 showed very little activity, and ASF/SF2 and 9G8 individually had intermediate activity. At least one SR protein (ASF/SF2) bound to the FP/ESE with characteristics of a cooperative interaction. Most interestingly, low concentrations of ASF/SF2 and 9G8 acted synergistically to activate intron D splicing. This was due in part to synergistic binding to the FP/ESE. Splicing of bGH intron D is inherently complex, and is likely controlled by an interaction of the FP/ESE with several trans-acting protein factors acting both independently and cooperatively. This level of complexity may be required for precise control of alternative splicing by an exon sequence, which simultaneously is constrained to maintain translational integrity of the mature mRNA.
Assuntos
Processamento Alternativo , Elementos Facilitadores Genéticos , Hormônio do Crescimento/genética , Íntrons/genética , Proteínas Nucleares/fisiologia , Proteínas de Transporte Nucleocitoplasmático , Precursores de RNA/metabolismo , Proteínas de Ligação a RNA/fisiologia , Animais , Sequência de Bases , Sítios de Ligação , Bovinos , Sequência Consenso , Globinas/genética , Humanos , Substâncias Macromoleculares , Dados de Sequência Molecular , Ligação Proteica , Proteínas Recombinantes de Fusão/metabolismo , Sequências Reguladoras de Ácido Nucleico , Fatores de Processamento de Serina-ArgininaRESUMO
The methylation of internal adenosine residues in eukaryotic mRNA, forming N6-methyladenosine (m6A), is catalyzed by a complex multicomponent enzyme. Previous studies suggested that m6A affects the efficiency of mRNA processing or transport, although the mechanism by which this occurs is not known. As a step toward better understanding the mechanism and function of this ubiquitous posttranscriptional modification, we have shown that HeLa mRNA (N6-adenosine)-methyltransferase requires at least two separate protein factors, MT-A and MT-B, and MT-A contains the AdoMet binding site on a 70-kDa subunit (MT-A70). MT-A70 was purified by conventional chromatography and electrophoresis, and was microsequenced. The peptide sequence was used to design a degenerate oligodeoxynucleotide that in turn was used to isolate the cDNA clone coding for MT-A70 from a HeLa cDNA library. Recombinant MT-A70 was expressed as a fusion protein in bacteria and was used to generate anti-MT-A70 antisera in rabbits. These antisera recognize MT-A70 in HeLa nuclear extracts by western blot and are capable of depleting (N6-adenosine)-methyltransferase activity from HeLa nuclear extract, confirming that MT-A70 is a critical subunit of (N6-adenosine)-methyltransferase. Northern blot analysis reveals that MT-A70 mRNA is present in a wide variety of human tissues and may undergo alternative splicing. MT-A70 cDNA probe hybridizes to a 2.0-kilobase (kb) polyadenylated RNA isolated from HeLa cells, whereas it hybridizes to two predominant RNA species (approximately 2.0 kb and 3.0 kb) using mRNA isolated from six different human tissues. Analysis of the cDNA sequence indicates that it codes for a 580-amino acid protein with a predicted MW = 65 kDa. The predicted protein contains sequences similar to consensus methylation motifs I and II identified in prokaryotic DNA (N6-adenosine)-methyltransferases, suggesting the functional conservation of peptide motifs. MT-A70 also contains a long region of homology to the yeast protein SPO8, which is involved in induction of sporulation by an unknown mechanism.
Assuntos
Metiltransferases/genética , Fragmentos de Peptídeos/genética , Sequência de Aminoácidos , Sequência de Bases , Northern Blotting , Western Blotting , Núcleo Celular/enzimologia , Cromatografia por Troca Iônica , Clonagem Molecular , DNA Complementar , Eletroforese em Gel de Poliacrilamida , Imunofluorescência , Células HeLa , Humanos , Metiltransferases/química , Metiltransferases/isolamento & purificação , Dados de Sequência Molecular , Fragmentos de Peptídeos/química , Fragmentos de Peptídeos/isolamento & purificação , Proteínas Recombinantes/química , Proteínas Recombinantes/genética , Proteínas Recombinantes/isolamento & purificação , Homologia de Sequência de Aminoácidos , Frações Subcelulares/enzimologiaRESUMO
N6-Methyladenosine is found at internal positions of mRNA in higher eukaryotes. This post-transcriptional modification occurs at a frequency of one to three methylation/average mRNA molecule in mammalian cell lines and is sequence-specific. A highly conserved consensus recognition site for the methyltransferase has been determined from both viral and cellular messages, consisting of the sequence Pu(G/A)AC(U/A) (with A being methylated). Despite the ubiquity and the specificity of this modification, little is known about the mechanism of formation of N6-methyladenosine. Utilizing an in vitro methylation system from HeLa cell nuclear extracts, and a substrate RNA derived from the mRNA coding for bovine prolactin, the mRNA N6-adenosine methyltransferase has been characterized and partially purified. Unique among other characterized nucleic acid methyltransferases, the enzyme is composed of three components which are separable under non-denaturing conditions. The molecular masses of the components are 30, 200, and 875 kDa as determined by gel filtration and glycerol gradient sedimentation. The 200-kDa component appears to contain the S-adenosylmethionine-binding site on a 70-kDa subunit. The 875-kDa component has affinity for single-stranded DNA-agarose, suggesting that it may contain the mRNA-binding site. N6-Adenosine methyltransferase is not sensitive to treatment with micrococcal nuclease, nor to immunodepletion using an anti-trimethylguanosine antibody, suggesting that it does not contain an essential RNA component.
Assuntos
Núcleo Celular/enzimologia , Metiltransferases/metabolismo , RNA Mensageiro/metabolismo , Animais , Sequência de Bases , Cromatografia em Gel , Cromatografia Líquida de Alta Pressão , Cromatografia por Troca Iônica , Eletroforese em Gel de Poliacrilamida , Células HeLa , Humanos , Metilação , Metiltransferases/isolamento & purificação , Dados de Sequência MolecularRESUMO
The N6-methylation of internal adenosine residues is a common post-transcriptional modification of eukaryotic pre-mRNA sequences. An in vitro methylation system which retains the precise selectivity of in vivo methylation sites has been used to further define the nature of RNA site recognition. In addition to short consensus sequences, other structural features or context effects contribute to the selection of methylation sites in pre-mRNAs. Partial purification of the mRNA N6-adenosine methyltransferase revealed unexpected levels of complexity. The methyltransferase is composed of three separate components with molecular masses of 30, 200 and 875 kDa, respectively. These components are readily separated under non-denaturing conditions and each is required for mRNA methylation activity.
