At the heart of transcription
Structure of the core of TFIID complex. Two copies of the different subunits of the complex (TAF4, TAF5, TAF6, TAF9 and TAF12) form a symmetrical structure. CREDIT: Gabor Papai.
Nature Jan. 31, 2013
Jan. 6, 2013
IGBMC researchers have revealed a very detailed structure of TFIID, a key molecule in the gene transcription process. These results obtained jointly with EMBL researchers are published on January 6th 2013 in Nature.
Gene expression takes place in two steps: DNA is first transcribed into RNA, and then the RNA is translated into proteins. In 2010, researchers in Patrick Schultz’s team at the IGBMC had already studied the molecular mechanisms of transcription initiation[ref] and had deciphered the molecular scenario of this process. Since then, their studies were focused on the heart of this machinery, the transcription factor TFIID and its structural organization.
Exquisite sample quality
While in their previous study, endogenous TFIID purified from yeast were employed, the researchers now solicited Imre Berger’s team at the EMBL in Grenoble, a specialist in the production of recombinant eukaryotic multiprotein complexes. The principle of their method is to express simultaneously all the proteins of the complex in insect cells with the help of a baculovirus. Once the genes are integrated in the virus, they are strongly expressed and produce large quantities of proteins. The production of human core of TFIID was particularly complicated, since it is composed of 5 different subunits, which entails to insert the same number of genes in a single virus. This collaboration enabled the teams of Patrick Schultz and Laszlo Tora to have access to perfectly homogenous and stable human samples.
More and more precision
The quality of these new samples was suitable to obtain high resolution images by cryo-electron microscopy. Digital image analysis revealed a pseudo-atomic model of TFIID. It was already known that this complex comprises 5 proteins: TAF4, TAF5, TAF6, TAF9 and TAF12, but their spatial organization and their precise position within the structure were unknown. The electron microscopy results enabled the determination of the global shape of core TFIID at a resolution of 10 Å. Thereafter, like a puzzle, the already known atomic structure of the 5 subunits have been placed into the global structure of TFIID in order to create the 3-dimensional model of the complex.
A tale of symmetry
In this study, researchers have shown that core TFIID was perfectly symmetrical, a result that scientists already suspected because of the presence of two copies of each of the five subunits. They further showed that when the two TAF8 and TAF10 subunits later join the core TFIID structure, they provoke a structural rearrangement that prevents the recruitment of a second set of these proteins thus breaking the symmetry of the complex. These results are unveiling the architecture of TFIID and its structural transition, from a symmetrical to an asymmetrical molecule, a crucial step in the assembly of the complex.