RESUMEN
Translation re-initiation provides the molecular basis for translational control of mammalian ATF4 and yeast GCN4 mediated by short upstream open reading (uORFs) in response to eIF2 phosphorylation. eIF4G is the major adaptor subunit of eIF4F that binds the cap-binding subunit eIF4E and the mRNA helicase eIF4A and is also required for re-initiation in mammals. Here we show that the yeast eIF4G2 mutations altering eIF4E- and eIF4A-binding sites increase re-initiation at GCN4 and impair recognition of the start codons of uORF1 or uORF4 located after uORF1. The increase in re-initiation at GCN4 was partially suppressed by increasing the distance between uORF1 and GCN4, suggesting that the mutations decrease the migration rate of the scanning ribosome in the GCN4 leader. Interestingly, eIF4E overexpression suppressed both the phenotypes caused by the mutation altering eIF4E-binding site. Thus, eIF4F is required for accurate AUG selection and re-initiation also in yeast, and the eIF4G interaction with the mRNA-cap appears to promote eIF4F re-acquisition by the re-initiating 40 S subunit. However, eIF4A overexpression suppressed the impaired AUG recognition but not the increase in re-initiation caused by the mutations altering eIF4A-binding site. These results not only provide evidence that mRNA unwinding by eIF4A stimulates start codon recognition, but also suggest that the eIF4A-binding site on eIF4G made of the HEAT domain stimulates the ribosomal scanning independent of eIF4A. Based on the RNA-binding activities identified within the unstructured segments flanking the eIF4G2 HEAT domain, we discuss the role of the HEAT domain in scanning beyond loading eIF4A onto the pre-initiation complex.
Asunto(s)
Factor 4A Eucariótico de Iniciación/metabolismo , Factor 4G Eucariótico de Iniciación/química , Factor 4G Eucariótico de Iniciación/metabolismo , Iniciación de la Cadena Peptídica Traduccional , Saccharomyces cerevisiae/enzimología , Factores de Transcripción con Cremalleras de Leucina de Carácter Básico/genética , Factores de Transcripción con Cremalleras de Leucina de Carácter Básico/metabolismo , Codón Iniciador/genética , Factor 4E Eucariótico de Iniciación/metabolismo , Regulación Fúngica de la Expresión Génica , Modelos Biológicos , Mutación/genética , Sistemas de Lectura Abierta/genética , Péptidos , Unión Proteica , Estructura Terciaria de Proteína , ARN de Hongos/metabolismo , ARN Mensajero/genética , ARN Mensajero/metabolismo , Saccharomyces cerevisiae/citología , Saccharomyces cerevisiae/genética , Proteínas de Saccharomyces cerevisiae/genética , Proteínas de Saccharomyces cerevisiae/metabolismo , Relación Estructura-ActividadRESUMEN
CONTEXT: We verified molecular alterations in a 72-year-old Brazilian male patient with a clinical course of homozygous beta-thalassemia intermedia, who had undergone splenectomy and was surviving without regular blood transfusions. The blood cell count revealed microcytic and hypochromic anemia (hemoglobin = 6.5 g/dl, mean cell volume = 74 fl, mean cell hemoglobin = 24 pg) and hemoglobin electrophoresis showed fetal hemoglobin = 1.3%, hemoglobin A2 = 6.78% and hemoglobin A = 79.4%. OBJECTIVE: To identify mutations in a patient with the symptoms of beta-thalassemia intermedia. DESIGN: Molecular inquiry into the mutations possibly responsible for the clinical picture described. SETTING: The structural molecular biology and genetic engineering center of the Universidade Estadual de Campinas, Campinas, Brazil. PROCEDURES: DNA extraction was performed on the patient's blood samples. The polymerase chain reaction (PCR) was done using five specific primers that amplified exons and the promoter region of the beta globin gene. The samples were sequenced and then analyzed via computer programs. RESULTS: Two mutations that cause the disease were found: -101 (C > T) and codon 39 (C > T). CONCLUSIONS: This case represents the first description of -101 (C > T) mutation in a Brazilian population and it is associated with a benign clinical course.
Asunto(s)
Globinas/genética , Mutación Puntual , Talasemia beta/genética , Anciano , Codón/genética , Humanos , Masculino , Reacción en Cadena de la PolimerasaRESUMEN
CONTEXT: We verified molecular alterations in a 72-year-old Brazilian male patient with a clinical course of homozygous beta-thalassemia intermedia, who had undergone splenectomy and was surviving without regular blood transfusions. The blood cell count revealed microcytic and hypochromic anemia (hemoglobin = 6.5 g/dl, mean cell volume = 74 fl, mean cell hemoglobin = 24 pg) and hemoglobin electrophoresis showed fetal hemoglobin = 1.3 percent, hemoglobin A2 = 6.78 percent and hemoglobin A = 79.4 percent. OBJECTIVE: To identify mutations in a patient with the symptoms of beta-thalassemia intermedia. DESIGN: Molecular inquiry into the mutations possibly responsible for the clinical picture described. SETTING: The structural molecular biology and genetic engineering center of the Universidade Estadual de Campinas, Campinas, Brazil. PROCEDURES: DNA extraction was performed on the patient's blood samples. The polymerase chain reaction (PCR) was done using five specific primers that amplified exons and the promoter region of the beta globin gene. The samples were sequenced and then analyzed via computer programs. RESULTS: Two mutations that cause the disease were found: -101 (C > T) and codon 39 (C > T). CONCLUSIONS: This case represents the first description of 101 (C > T) mutation in a Brazilian population and it is associated with a benign clinical course