MBE Advance Access originally published online on April 17, 2007
Molecular Biology and Evolution 2007 24(7):1458-1463; doi:10.1093/molbev/msm073
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Research Articles |
Compensatory Change of Interacting Amino Acids in the Coevolution of Transcriptional Coactivator MBF1 and TATA-BoxBinding Protein
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* Center for Information Biology and DNA Data Bank of Japan, National Institute of Genetics, Mishima, Shizuoka, Japan
Department of Developmental Genetics, National Institute of Genetics, Mishima, Shizuoka, Japan
E-mail: tgojobor{at}genes.nig.ac.jp.
| Abstract |
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To elucidate the transcriptional regulation in eukaryotic genome network, it is important to understand coevolution of transcription factors, transcriptional coactivators, and TATA-boxbinding protein (TBP). In this study, coevolution of transcriptional coactivator multiprotein-bridging factor 1 and its interacting target TBP was first evaluated experimentally by examining if compensatory amino acid changes took place at interacting sites of both proteins. The experiments were conducted by identifying interaction sites and comparing the amino acids at these sites among different organisms. Here, we provide evidence for compensatory changes of transcription coactivator and its interacting target, presenting the 1st report that transcription coactivator may have undergone coevolution with TBP.
Key Words: compensatory change coevolution transcriptional coactivator MBF1 TBP
| Introduction |
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Transcription factors are components of the regulatory network and are involved in multiple interactions with other proteins. Thus, the evolution of transcription factor families takes place within a framework defined by these interactions. Because the conserved domains within transcription factors often contain sites that mediate these interactions, their conservation most likely reflects the conservation of classes of interactions that were established early in evolution and under the limitations of tolerable (i.e., functional) changes. Many transcription factors have been evolutionarily conserved (Ge et al. 2002
| Materials and Methods |
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Polymerase Chain Reaction Mutagenesis
The tbp (TBP genecontaining mutations) library was made by error-prone polymerase chain reaction (PCR) as described (Lin-Goerke et al. 1997
Screening for tbp Genes
The Saccharomyces cerevisiae yeast strain used for screening was N107-1 (MATa ade2-1 ura3-1 trp1-1 leu2-3, 112 can1-100
tbp::LEU2 [Ycplac33-yTBP]). This strain has chromosomal TBP gene deletion replaced by an URA3-marked plasmid carrying the TBP gene. The tbp library was transformed into N107-1 and spread onto plates with synthetic complete (SC) media but did not contain tryptophan. Strains expressing tbp were grown on 5-fluoroorotic acid (5-FOA) to remove plasmid-carrying TBP and then shifted to submaster plates either containing aminotriazole (AT) or not containing AT. We screened strains that were AT sensitive but showed normal growth.
Glutathione S-transferase Pull-Down Assay
Q68L and Q68I tbp genes were subcloned into 6HisT-pET11d to produce TBP proteins bearing 6 histidine residues in Escherichia coli. His-tagged recombinant proteins were purified using a Ni-column (Novagen, San Diego) and used for assay. The Glutathione S-transferase (GST) pull-down assay was performed using GSTMBF1 as described (Takemaru et al. 1998
). Bound proteins were detected on western blots using an anti-TBP antibody.
Protein Sequence Analysis
All available sequences were obtained using the Entrez Protein database at National Center for Biotechnology Information (NCBI) (http://www.ncbi.nlm.nih.gov/). Accession numbers and species were compiled in a supplementary table S1 (Supplementary Material online). Protein-coding sequences were aligned using the ClustalX program (Thompson et al. 1997
). All amino acid positions with gaps were excluded from this analysis. For phylogenetic reconstruction of TBP (Supplementary fig. 1, Supplementary Material online), the Neighbor-Joining method was used (Saitou and Nei 1987
) with observed differences as implemented Njplot (Perriere and Gouy 1996
). Bootstrap analysis with 1,000 replicates was used to assess the support for tree nodes (Felsenstein 1985
). Phylogenetic distribution of interaction amino acids between MBF1 and TBP is based on phylogenetic analysis of full-length TBP sequences and recent studies (Hedges 2002
).
Compensatory Change Analysis
The S. cerevisiae yeast strain used for compensatory change of interaction amino acid analysis was N111-4A (ade2-1 ura3-1 trp-1 leu2-3, 112 can1-100
mbf1::LEU2
tbp::LEU2 [yCplac33-TBP]). Ycplac22-TBP and Ycplac22-tbp mutant plasmids were transformed into N111-4A and spread onto plates with SC media but do not contain tryptophan. Strains expressing tbp were grown on 5-FOA to remove plasmid-carrying TBP. Ycplac33-MBF1 or Ycplac33-mbf1 mutant plasmids were then transformed into strains and spread onto plates with SC media but not contain Uracil. These strains were then shifted to submaster plates either containing AT or not containing AT. Point mutations of MBF1 and TBP were introduced by site-directed PCR mutagenesis.
| Results |
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Q68 of yTBP Is Required for yMBF1 Binding
MBF1 and TBP sequences are evolutionarily conserved from Archaea to humans (supplementary table S1, Supplementary Material online). To obtain evidence for the coevolution of MBF1 and TBP, we identified amino acids involved in the interaction between MBF1 and TBP in the yeast S. cerevisiae. It has been shown that D112 of yMBF1 is necessary for the yTBP binding (Takemaru et al. 1998
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Alignment Analysis of MBF1 and TBP
To understand how interacting amino acids evolve, we next analyzed the sequences of MBF1 and TBP of various organisms (figs. 2 and 3). Archaeal MBF1 contains a Zn-ribbon motif that is absent in their eukaryotic counterparts. Eukaryotic MBF1 consists of 2 structural domains; a well-structured C-terminal half that binds to TBP and a flexible N-terminal half that participates in binding to various activators (Ozaki et al. 1999
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Compensatory Change Analysis In Vivo
To confirm the compensatory change of interacting amino acids, we did an in vivo analysis of interacting amino acids of MBF1 and TBP in the yeast S. cerevisiae. We made mutants of MBF1 and TBP according to the results of the evolutionary analysis (fig. 4; Supplementary fig. 2, Supplementary Material online). The mutants TBP-68Q, MBF1-112K and TBP-68E, MBF1-112D were sensitive to 3AT (fig. 5), indicating that the interactions between MBF1 and TBP were disrupted in these mutants. As expected from the evolutionary analysis (fig. 4), the mutants TBP-68Q, MBF1-112R; TBP-68E, MBF1-112R; TBP-68E, MBF1-112N; TBP-68H, MBF1-112E; and TBP-68E, MBF1-112K were resistant to AT (fig. 5). These results suggest that the compensatory change occurred and was selected from the neutral mutations during the evolution of MBF1 and TBP.
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| Discussion |
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Coevolution of MBF1 and TBP
Coevolution is a process in which an inheritable change in one entity exerts selective pressure for a change in another entity. The coevolution of proteins has been well studied (Pazos et al. 1997
Why Is Interaction between MBF1 and TBP Conserved?
MBF1 is conserved among all organisms in which TBP is used as the general transcription factor. The coactivator is preserved even in a parasitic protozoan Cryptosporidium parvum where many essential genes are lost from its genome and their functions are supplied by the host counterparts (Abrahamsen et al. 2004
). This study demonstrated the coevolution of MBF1 and the essential protein TBP. All these findings suggest the importance of MBF1. Nevertheless, null mutants of MBF1 are viable in both yeast and Drosophila under laboratory conditions. Does this contradict the neutral theory of evolution (Kimura 1955
), which predicts the importance of conserved genes? The answer is no. Recently, studies revealed diverse biological function of MBF1. Yeast MBF1 supports the GCN4-dependent activation of the HIS3 gene (Takemaru et al. 1998
), and Drosophila MBF1 serves as a coactivator of basic leucine Zipper protein Tracheae defective during morphogenesis of the tracheal and nervous systems (Liu et al. 2003
). Drosophila MBF1 also interacts with AP-1 to preserve redox-dependent AP-1 activity during oxidative stress (Jindra et al. 2004
). Rat MBF1 has been isolated as a calmodulin-associated peptide 19 (Smith et al. 1998
), and human MBF1 has been identified as endothelial differentiationrelated factor 1 (Mariotti et al. 2000
). Tomato MBF1 is induced immediately and transiently in ethylene-treated late immature fruit (Zegzouti et al. 1999
). Potato MBF1 is upregulated during fungal attack, upon wounding, and by treatment with salicylic acid and the ethylene precursor ethephon (Godoy et al. 2001
). Tobacco MBF1 is induced by the combined effect of drought stress and heat shock (Rizhsky et al. 2002
). Therefore, the interaction between MBF1 and TBP appears to be essential in the real world where organisms are subject to nutrient starvation and various kinds of stresses, and proper differentiation timing is critical for life.
| Supplementary Material |
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Supplementary table S1 and Supplementary figures 1 and 2 are available at Molecular Biology and Evolution online (http://www.mbe.oxfordjournals.org/).
| Acknowledgements |
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We thank Dr Naruya Saitou and Dr Yoshiyuki Suzuki for helpful discussions. This work was supported by Grants-in-aid for Scientific Research from the Ministry of Education, Science, Sports, Culture and Technology of Japan.
| Footnotes |
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1 Equal contribution to this work.
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