Chaperone-mediated ordered assembly of the SAGA transcription complex

Transcription initiation involves the coordinated activities of large multimeric complexes. Little is known about the mechanisms and pathways that drive their assembly from individual components. We report here several principles underlying the assembly of the highly conserved SAGA co-activator complex, which is composed of 19 subunits organized into functional modules. First, we demonstrate that SAGA assembles through an ordered pathway, in which the core subunit Spt20 recruits Tra1, which then promotes the incorporation of the de-ubiquitination module (DUB). Second, affinity purifications and phenotypic analyses identified a small region of Spt20 that is both necessary and sufficient to anchor Tra1 to SAGA. Third, Tra1 is shared with the NuA4 co-activator complex and is the only pseudokinase of the PIKK family. We accumulated functional and biochemical evidence that Tra1 shares a specific Hsp90 cochaperone with PIKKs, the Triple-T complex, for its cotranslational folding and de novo incorporation into both SAGA and NuA4. Finally, in contrast to its specific role in SAGA assembly, Tra1 contributes to scaffold the entire NuA4 complex. Overall, our study brings mechanistic insights into the de novo assembly of transcriptional complexes via ordered pathways and reveals the contribution of dedicated chaperones to this process.