Architecture of the Saccharomyces cerevisiae SAGA transcription coactivator complex
The evolutionarily conserved transcription coactivator SAGA (Spt-Ada-Gcn5 Acetyltransferase) is a multi-subunit complex with a modular structure, which has several distinct activities that are used to regulate activator-dependent transcription. SAGA covalently modifies histones using its histone acetyltransferase (HAT) and deubiquitination (DUB) modules. It also directly regulates the formation of the transcription preinitiation complex (PIC) through direct interactions with both transcriptional activators and the TATA-box Binding Protein (TBP). Despite SAGA's important roles in regulating transcription, its overall architecture and structural organization of its modules remain unclear. The large size and complex subunit composition of SAGA make it difficult to study its structure using high-resolution approaches such as X-ray crystallography. Using an alternative approach, I combined chemical crosslinking with mass spectrometry (CXMS) to investigate the architecture of SAGA. In Chapter 2, I describe the results of my efforts using this approach, finding that the SAGA Taf and Taf-like subunits form a TFIID-like core complex at the center of SAGA that makes extensive interactions with all other SAGA modules. In Chapter 3, I show that the HAT and DUB modules are in close proximity, and the DUB module modestly stimulates HAT function. In Chapter 4, I describe the finding that SAGA-TBP binding involves a network of interactions between subunits Spt3, Spt8, Spt20, and Spt7, and the attempts I have made toward solving the crystal structure of Spt8 in complex with TBP. Finally, in Chapter 5, I combine all of the data and derive a model for the molecular architecture of the SAGA complex. My results provide new insight into SAGA function in gene regulation, its structural similarity with TFIID, and functional interactions between the SAGA modules.
- Biological chemistry