Transcription activation domains of the yeast factors Met4 and Ino2: tandem activation domains with properties similar to the yeast Gcn4 activator

Eukaryotic transcription activation domains (ADs) are intrinsically disordered polypeptides that typically interact with coactivator complexes, leading to stimulation of transcription initiation, elongation and chromatin modifications. Here we examine the properties of two strong and conserved yeast ADs: Met4 and Ino2. Both factors have tandem ADs that were identified by conserved sequence and functional studies. While AD function from both factors depends on hydrophobic residues, Ino2 further requires key conserved acidic and polar residues for optimal function. Binding studies show that the ADs bind multiple Med15 activator binding domains (ABDs) with a similar order of micromolar affinity, and similar but distinct thermodynamic properties. Protein crosslinking shows that no unique complex is formed upon Met4-Med15 binding. Rather, we observed heterogeneous AD-ABD contacts with nearly every possible AD-ABD combination. Many of these properties are similar to those observed with the yeast activator Gcn4, which forms a large heterogeneous, dynamic, and fuzzy complex with Med15. We suggest that this molecular behavior is common among eukaryotic activators.