Molecular evolution is driven by mutations, which may affect the fitness of an organism and are then subject to natural selection or genetic drift. Analysis of primary protein sequences and tertiary structures has yielded valuable insights into the evolution of protein function, but little is known about evolution of functional mechanisms, protein dynamics and conformational plasticity essential for activity. We characterized the atomic-level motions across divergent members of the dihydrofolate reductase (DHFR) family. Despite structural similarity, E. coli and human DHFRs use different dynamic mechanisms to perform the same function, and human DHFR cannot complement DHFR-deficient E. coli cells. Identification of the primary sequence determinants of flexibility in DHFRs from several species allowed us to propose a likely scenario for the evolution of functionally important DHFR dynamics, following a pattern of divergent evolution that is tuned by the cellular environment.

Although many transcription activators contact the same set of coactivator complexes, the mechanism and specificity of these interactions have been unclear. For example, do intrinsically disordered transcription activation domains (ADs) use sequence-specific motifs, or do ADs of seemingly different sequence have common properties that encode activation function? We find that the central activation domain (cAD) of the yeast activator Gcn4 functions through a short, conserved sequence-specific motif. Optimizing the residues surrounding this short motif by inserting additional hydrophobic residues creates very powerful ADs that bind the Mediator subunit Gal11/Med15 with high affinity via a "fuzzy" protein interface. In contrast to Gcn4, the activity of these synthetic ADs is not strongly dependent on any one residue of the AD, and this redundancy is similar to that of some natural ADs in which few if any sequence-specific residues have been identified. The additional hydrophobic residues in the synthetic ADs likely allow multiple faces of the AD helix to interact with the Gal11 activator-binding domain, effectively forming a fuzzier interface than that of the wild-type cAD.Mediator complex | protein NMR T ranscription activators are regulators of cell identity, cell growth, and the response to environmental conditions. These highly regulated factors contain one or more activation domains (ADs) that typically bind coactivators-the complexes that contact the transcription machinery and/or have chromatin modifying activity (1-5). AD-coactivator binding initiates a cascade of events leading to productive transcription including targeted chromatin remodeling and stimulation of both RNA polymerase II preinitiation complex formation and transcription elongation (6). Many broadly acting ADs bind several coactivators, allowing them to function at a wide range of promoters with different coactivator requirements (7)(8)(9)(10)(11)(12)(13)(14). The function of most tested ADs is conserved among eukaryotes (15, 16), even though some key activator targets are not conserved. For example, the herpes virus protein VP16 strongly activates transcription in both yeast and mammalian cells, although human Med25, a critical target of VP16 in humans, is not found in yeast (17,18). Thus, the ability to adapt to different coactivator targets is seemingly a key property of ADs.Defining what constitutes a functional AD has been difficult, because there is little apparent sequence similarity among different ADs. All structurally characterized eukaryotic ADs lack a stable 3D structure and are disordered in the absence of a coactivator target (19)(20)(21)(22)(23)(24)(25). Initial studies classified ADs based on enriched residues: acidic proline-, glutamine-, or serine-rich activators (26). However, these residue types later were found to be generally enriched in intrinsically disordered proteins and were termed "disorder-promoting residues" (27, 28).Attempts to define functional sequence motifs within ADs have led to ambiguous results. For many ...

Summary Transcription activation domains (ADs) are inherently disordered proteins that often target multiple coactivator complexes, but the specificity of these interactions is not understood. Efficient transcription activation by yeast Gcn4 requires its tandem ADs and four activator-binding domains (ABDs) on its target, the Mediator subunit Med15. Multiple ABDs are a common feature of coactivator complexes. We find that the large Gcn4-Med15 complex is heterogeneous and contains nearly all possible AD-ABD interactions. Gcn4-Med15 forms via a dynamic fuzzy protein-protein interface, where ADs bind the ABDs in multiple orientations via hydrophobic regions that gain helicity. This combinatorial mechanism allows individual low-affinity and specificity interactions to generate a biologically functional, specific, and higher affinity complex despite lacking a defined protein-protein interface. This binding strategy is likely representative of many activators that target multiple coactivators, as it allows great flexibility in combinations of activators that can cooperate to regulate genes with variable coactivator requirements.