The architecture of human general transcription factor TFIID core complex — Schaffitzel and Berger labs (2013)
Until now, the inability to produce enough high-quality recombinant complex has hindered scientists from obtaining the structural information necessary to fully understand the molecular function of the general transcription factor TFIID. Several groups led by the Berger team have now elucidated a high-resolution structure (11.6Å) for the core TFIID complex, a 650 kDa complex containing ten protein subunits. Their findings, published in Nature, now reveal the architecture of the core TFIID and answer several long-standing questions about TFIID.
This breakthrough was due in a large part to the ability to produce large quantities of very high-quality complex that was properly assembled and had the correct stoichiometry. Key to this was the MultiBac protein expression system produced recombinant in insect cells with a baculovirus; here, the proteins were expressed as polyproteins. This protein production method will be critical for expressing the PRC and NuRD complexes in the 4DCellFate project.
C. Bieniossek, G. Papai, C. Schaffitzel, F. Garzoni, M. Chaillet, E. Scheer, P. Papadopoulos, L. Tora, P. Schultz, I. Berger
The initiation of gene transcription by RNA polymerase II is regulated by a plethora of proteins in human cells. The first general transcription factor to bind gene promoters is transcription factor IID (TFIID). TFIID triggers pre-initiation complex formation, functions as a coactivator by interacting with transcriptional activators and reads epigenetic marks. TFIID is a megadalton-sized multiprotein complex composed of TATA-box-binding protein (TBP) and 13 TBP-associated factors (TAFs). Despite its crucial role, the detailed architecture and assembly mechanism of TFIID remain elusive. Histone fold domains are prevalent in TAFs, and histone-like tetramer and octamer structures have been proposed in TFIID. A functional core-TFIID subcomplex was revealed in Drosophila nuclei, consisting of a subset of TAFs (TAF4, TAF5, TAF6, TAF9 and TAF12). These core subunits are thought to be present in two copies in holo-TFIID, in contrast to TBP and other TAFs that are present in a single copy, conveying a transition from symmetry to asymmetry in the TFIID assembly pathway. Here we present the structure of human core-TFIID determined by cryo-electron microscopy at 11.6A ̊ resolution. Our structure reveals a two-fold symmetric, interlaced architecture, with pronounced protrusions, that accommodates all conserved structural features of the TAFs including the histone folds. We further demonstrate that binding of one TAF8–TAF10 complex breaks the original symmetry of core-TFIID. We propose that the resulting asymmetric structure serves as a functional scaffold to nucleate holo-TFIID assembly, by accreting one copy each of the remaining TAFs and TBP.
Nature (2013) 493:699-702; doi:10.1038/nature11791; Pubmed