Final Report Summary - COACTIVATOR (Regulation of gene expression by transcriptional coactivators) How cells respond to external changes by regulating gene expression is a fundamental question in biology. Our first objective is to better understand how cells allow both coordination and versatility in gene expression. Gene regulation can occur at many distinct steps. One critical point of control is transcription, which is regulated by many chromatin modifiers and remodelers, including the SAGA co-activator complex. We showed that, in fission yeast, SAGA regulates gene expression in response to a change in nutrient levels, downstream of both the TORC1 and TORC2 signaling pathways. We then established that the Taf12 subunit of SAGA becomes transiently phosphorylated upon starvation. This event is controlled by the opposing activities of the PP2A phosphatase, which is activated by TORC1, and the Gad8AKT kinase, which is activated by TORC2. Importantly, Taf12 phosphorylation functions to buffer the commitment to differentiation early upon starvation. Overall, our work reveals that SAGA is a direct target of nutrient-sensing pathways and has uncovered a mechanism by which TORC1 and TORC2 converge to control gene expression and cell fate decisions.Our second objective is to address how chromatin regulatory complexes are assembled and whether their assembly can be modulated to control their activities, focusing on the highly conserved SAGA co-activator. Its largest subunit, Tra1, is required for recruiting SAGA to promoters. Tra1 is a member of the PIKK family of kinases, but lacks catalytic residues. Recent work has established that PIKKs are activated by a novel chaperone, TTT. We accumulated biochemical and functional evidence that Tra1, although a pseudo-kinase, is assembled into SAGA by TTT. We identified key residues within SAGA and Tra1 that mediate their interaction and the domain of Tra1 which is recognized by TTT for its folding. Interestingly, this domain is conserved between all PIKKs, suggesting a shared molecular mechanism of assembly. Finally, we showed that TTT recognizes nascent, unfolded PIKKs, to catalyze their folding and incorporation into active complexes.In conclusion, our work has uncovered a previously unknown mechanism for the regulation of transcription by signaling pathways and strengthen an emerging concept in the field of signal transduction and gene regulation, which is that specific chaperones control the assembly of multi-protein complexes to coordinate their activities.
