ABSTRACT
Ribosome anti-association factor eIF6 (originally named according to translation initiation terminology as eukaryotic initiation factor 6) binds to the large ribosomal subunit, thereby preventing inappropriate interactions with the small subunit during initiation of protein synthesis. We have determined the X-ray structures of two IF6 homologs, Methanococcus jannaschii archaeal aIF6 and Sacchromyces cerevisiae eIF6, revealing a phylogenetically conserved 25 kDa protein consisting of five quasi identical alpha/beta subdomains arrayed about a five-fold axis of pseudosymmetry. Yeast eIF6 prevents ribosomal subunit association. Comparative protein structure modeling with other known archaeal and eukaryotic homologs demonstrated the presence of two conserved surface regions, one or both of which may bind the large ribosomal subunit.
Subject(s)
Methanococcus/chemistry , Peptide Initiation Factors/chemistry , Peptide Initiation Factors/metabolism , Ribosomes/chemistry , Ribosomes/metabolism , Saccharomyces cerevisiae/chemistry , Amino Acid Sequence , Conserved Sequence , Crystallography, X-Ray , Models, Molecular , Molecular Sequence Data , Protein Binding , Protein Structure, Quaternary , Protein Structure, Secondary , Protein Structure, Tertiary , Protein Subunits , Sequence Alignment , Water/metabolismABSTRACT
The three-dimensional structure of the human Rap30 DNA-binding domain has been solved by multinuclear NMR spectroscopy. The structure of the globular domain is strikingly similar to that of linker histone H5 and its fold places Rap30 into the "winged" helix-turn-helix family of eukaryotic transcription factors. Although the domain interacts weakly with DNA, the binding surface was identified and shown to be consistent with the structure of the HNF-3/fork head-DNA complex. The architecture of the Rap30 DNA-binding domain has important implications for the function of Rap30 in the assembly of the preinitiation complex. In analogy to the function of linker histones in chromatin formation, the fold of the Rap30 DNA-binding domain suggests that its role in transcription initiation may be that of a condensation factor for preinitiation complex assembly. Functional similarity to linker histones may explain the dependence of Rap30 binding on the bent DNA environment induced by the TATA box-binding protein. Cryptic sequence identity and functional homology between the Rap30 DNA-binding domain and region 4 of Escherichia coli sigma70 may indicate that the sigma factors also possess a linker histone-like activity in the formation of a prokaryotic closed complex.
