The strength of acidic activation domains correlates with their affinity for both transcriptional and non-transcriptional proteins.
Activation domains (ADs) appear to work by making specific protein-protein contacts with the transcriptional machinery. However, ADs show no apparent sequence conservation, they can be functionally replaced by a number of random peptides and unrelated proteins, and their function does not depend on sustaining a complex tertiary structure. To gain a broader perspective on the nature of interactions between acidic ADs and several of their proposed targets, the in vivo strengths of viral, human, yeast, and artificial activation domains were determined under physiological conditions, and mutant ADs with increased in vivo potencies were selected. The affinities between ADs and proposed targets were determined in vitro and all interactions were found to be of low-level affinity with dissociation constants above 10(-7)M. However, in vivo potencies of all ADs correlated nearly perfectly with their affinities for transcriptional proteins. Surprisingly, the weak interactions of the different ADs with at least two non-transcriptional proteins show the same rank order of binding and AD mutants selected for increased in vivo strength also have increased affinities to non-transcriptional proteins. Based on these results, isolated acidic ADs can bind with relatively low-level specificity and affinity to many different proteins and the strength of these semi-specific interactions determine the strength of an AD. I suggest that ADs expose flexible hydrophobic elements in an aqueous environment to contact hydrophobic patches over short distances, shifting specificity of activators largely to the DNA colocalization of arrays of ADs and targets.
Melcher
K
K
Departments of Internal Medicine and Biochemistry, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX, 75235-8573, USA. K.Melcher@em.uni-frankfurt.de
D000251:Adenosine Triphosphatases; D000595:Amino Acid Sequence; D000818:Animals; D002138:Calibration; D004268:DNA-Binding Proteins / Q000378:metabolism; D005656:Fungal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017405:Herpes Simplex Virus Protein Vmw65 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006801:Humans; D007526:Isoelectric Point; D008954:Models, Biological; D008969:Molecular Sequence Data; D009113:Muramidase / Q000378:metabolism*; D009154:Mutation / Q000235:genetics; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding; D011494:Protein Kinases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins / Q000378:metabolism*; D012441:Saccharomyces cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D013379:Substrate Specificity; D035381:TATA-Binding Protein Associated Factors; D035181:TATA-Box Binding Protein; D013816:Thermodynamics; D035581:Transcription Factor TFIIB; D035362:Transcription Factor TFIID; D014157:Transcription Factors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation; D016159:Tumor Suppressor Protein p53 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
D004268:DNA-Binding Proteins; D005656:Fungal Proteins; C052130:GAL4 protein, S cerevisiae; C055002:GAL80 protein, S cerevisiae; D017405:Herpes Simplex Virus Protein Vmw65; D011993:Recombinant Fusion Proteins; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; C105463:TAF11 protein, S cerevisiae; D035381:TATA-Binding Protein Associated Factors; D035181:TATA-Box Binding Protein; D035581:Transcription Factor TFIIB; D035362:Transcription Factor TFIID; D014157:Transcription Factors; D016159:Tumor Suppressor Protein p53; D011494:Protein Kinases; D009113:Muramidase; D046988:Proteasome Endopeptidase Complex; D000251:Adenosine Triphosphatases; C069670:RPT6 protein, S cerevisiae