RESUMEN
Eukaryotic initiation factor 1A (eIF1A) and the GTPase IF2/eIF5B are the only universally conserved translation initiation factors. Recent structural, biochemical and genetic data indicate that these two factors form an evolutionarily conserved structural and functional unit in translation initiation. Based on insights gathered from studies of the translation elongation factor GTPases, we propose that these factors occupy the aminoacyl-tRNA site (A site) on the ribosome, and promote initiator tRNA binding and ribosomal subunit joining. These processes yield a translationally competent ribosome with Met-tRNA in the ribosomal peptidyl-tRNA site (P site), base-paired to the AUG start codon of a mRNA.
Asunto(s)
Factor 1 Eucariótico de Iniciación , Iniciación de la Cadena Peptídica Traduccional , Factores de Iniciación de Péptidos/fisiología , Ribosomas/metabolismo , Proteínas Bacterianas/química , Proteínas Bacterianas/fisiología , Sitios de Unión , Factor 5 Eucariótico de Iniciación , Evolución Molecular , Guanosina Trifosfato/metabolismo , Humanos , Modelos Moleculares , Factores de Iniciación de Péptidos/química , Factores de Iniciación de Péptidos/genética , Estructura Terciaria de Proteína , ARN de Transferencia de Metionina/metabolismo , Alineación de SecuenciaAsunto(s)
Factor 2 Eucariótico de Iniciación/genética , Factores Eucarióticos de Iniciación/genética , Iniciación de la Cadena Peptídica Traduccional/genética , Biosíntesis de Proteínas/genética , Secuencia de Aminoácidos , Animales , Sitios de Unión , Secuencia Conservada , Factor 2 Eucariótico de Iniciación/química , Factor 2 Eucariótico de Iniciación/metabolismo , Factores Eucarióticos de Iniciación/química , Factores Eucarióticos de Iniciación/metabolismo , Guanosina Trifosfato/metabolismo , Methanobacterium/genética , Modelos Moleculares , Factor 2 Procariótico de Iniciación/genética , Factor 2 Procariótico de Iniciación/metabolismo , Conformación Proteica , Ribosomas/genética , Ribosomas/metabolismoRESUMEN
X-ray structures of the universal translation initiation factor IF2/eIF5B have been determined in three states: free enzyme, inactive IF2/eIF5B.GDP, and active IF2/eIF5B.GTP. The "chalice-shaped" enzyme is a GTPase that facilitates ribosomal subunit joining and Met-tRNA(i) binding to ribosomes in all three kingdoms of life. The conserved core of IF2/eIF5B consists of an N-terminal G domain (I) plus an EF-Tu-type beta barrel (II), followed by a novel alpha/beta/alpha-sandwich (III) connected via an alpha helix to a second EF-Tu-type beta barrel (IV). Structural comparisons reveal a molecular lever, which amplifies a modest conformational change in the Switch 2 region of the G domain induced by Mg(2+)/GTP binding over a distance of 90 A from the G domain active center to domain IV. Mechanisms of GTPase function and ribosome binding are discussed.
Asunto(s)
Guanosina Difosfato/metabolismo , Guanosina Trifosfato/metabolismo , Factores de Iniciación de Péptidos/química , Secuencias de Aminoácidos , Secuencia de Aminoácidos , Sitios de Unión , Cristalografía por Rayos X , Activación Enzimática , Escherichia coli/metabolismo , Factor 5 Eucariótico de Iniciación , Guanina/metabolismo , Methanococcus/química , Modelos Moleculares , Datos de Secuencia Molecular , Factores de Iniciación de Péptidos/metabolismo , Conformación Proteica , Estructura Secundaria de Proteína , Estructura Terciaria de Proteína , Homología de Secuencia de AminoácidoRESUMEN
To initiate protein synthesis, a ribosome with bound initiator methionyl-tRNA must be assembled at the start codon of an mRNA. This process requires the coordinated activities of three translation initiation factors (IF) in prokaryotes and at least 12 translation initiation factors in eukaryotes (eIF). The factors eIF1A and eIF5B from eukaryotes show extensive amino acid sequence similarity to the factors IF1 and IF2 from prokaryotes. By a combination of two-hybrid, coimmunoprecipitation, and in vitro binding assays eIF1A and eIF5B were found to interact directly, and the eIF1A binding site was mapped to the C-terminal region of eIF5B. This portion of eIF5B was found to be critical for growth in vivo and for translation in vitro. Overexpression of eIF1A exacerbated the slow-growth phenotype of yeast strains expressing C-terminally truncated eIF5B. These findings indicate that the physical interaction between the evolutionarily conserved factors eIF1A and eIF5B plays an important role in translation initiation, perhaps to direct or stabilize the binding of methionyl-tRNA to the ribosomal P site.
Asunto(s)
Proteínas Bacterianas/fisiología , Células Eucariotas/metabolismo , Factor 1 Eucariótico de Iniciación/fisiología , Factor 2 Eucariótico de Iniciación/fisiología , Iniciación de la Cadena Peptídica Traduccional/fisiología , Factores de Iniciación de Péptidos/fisiología , Células Procariotas/metabolismo , Escherichia coli/genética , Factor 5 Eucariótico de Iniciación , Sustancias Macromoleculares , Imitación Molecular , Factores de Iniciación de Péptidos/química , Factores de Iniciación de Péptidos/genética , Fenotipo , Factor 1 Procariótico de Iniciación , Unión Proteica , Estructura Terciaria de Proteína , ARN de Transferencia/genética , ARN de Transferencia de Metionina/genética , ARN de Transferencia de Metionina/metabolismo , Proteínas Recombinantes de Fusión/fisiología , Ribosomas/metabolismo , Saccharomyces cerevisiae/citología , Saccharomyces cerevisiae/genética , Saccharomyces cerevisiae/metabolismo , Especificidad de la Especie , Relación Estructura-Actividad , Técnicas del Sistema de Dos HíbridosRESUMEN
Binding of initiator methionyl-tRNA to ribosomes is catalyzed in prokaryotes by initiation factor (IF) IF2 and in eukaryotes by eIF2. The discovery of both IF2 and eIF2 homologs in yeast and archaea suggested that these microbes possess an evolutionarily intermediate protein synthesis apparatus. We describe the identification of a human IF2 homolog, and we demonstrate by using in vivo and in vitro assays that human IF2 functions as a translation factor. In addition, we show that archaea IF2 can substitute for its yeast homolog both in vivo and in vitro. We propose a universally conserved function for IF2 in facilitating the proper binding of initiator methionyl-tRNA to the ribosomal P site.