Transcription factor II D (TFIID) is a multiprotein complex that nucleates formation of the basal transcription machinery. TATA binding protein-associated factors 1 and 7 (TAF1 and TAF7), two subunits of TFIID, are integral to the regulation of eukaryotic transcription initiation and play key roles in preinitiation complex (PIC) assembly. Current models suggest that TAF7 acts as a dissociable inhibitor of TAF1 histone acetyltransferase activity and that this event ensures appropriate assembly of the RNA polymerase IImediated PIC before transcriptional initiation. Here, we report the 3D structure of a complex of yeast TAF1 with TAF7 at 2.9 Å resolution. The structure displays novel architecture and is characterized by a large predominantly hydrophobic heterodimer interface and extensive cofolding of TAF subunits. There are no obvious similarities between TAF1 and known histone acetyltransferases. Instead, the surface of the TAF1-TAF7 complex contains two prominent conserved surface pockets, one of which binds selectively to an inhibitory trimethylated histone H3 mark on Lys27 in a manner that is also regulated by phosphorylation at the neighboring H3 serine. Our findings could point toward novel roles for the TAF1-TAF7 complex in regulation of PIC assembly via reading epigenetic histone marks.initiation complex | protein structure | protein-protein interaction | X-ray crystallography T he general transcription factor II D (TFIID) plays a central role in recognition of the core promoter element and mediates accurate transcription initiation by RNA Polymerase (Pol) II for a large class of genes. TFIID nucleates the formation of the preinitiation complex (PIC) at the transcriptional start site by recruiting other general transcription factors (TFII-A, -B, -E, -F, and -H), along with RNA Pol II. TFIID is assembled in a stepwise manner, with a symmetric TFIID core complex recruited first, followed by further recruitment of additional TATA binding protein (TBP)-associated factors (TAFs) to form the complete asymmetric holo-TFIID complex (1). This megadalton-sized multiprotein assembly, comprised of TBP and 13 evolutionary conserved TAFs (2), is organized into a trilobed structure (3) and undergoes striking rearrangements upon binding to TFIIA and DNA (4). TAF subunits serve multiple functions within the TFIID holocomplex. In addition to TBP, TAF1, TAF2, TAF6, and TAF9 are also involved in recognition of DNA initiator and promoter elements. Moreover, TFIID can behave as an epigenetic effector, capable of recognizing posttranslational histone modifications associated with activated transcription. Eukaryotic TAF1 contains a double bromodomain that recognizes acetylated histones, and TAF3 contains a plant homeo domain (PHD) that binds to histone H3 methylated at lysine 4 (5, 6). In addition to roles in basal transcription, TFIID is also associated with diseases. Overexpression of TAF1, which acts as a specific coactivator of androgen receptor, is related to the progression of human prostate cancer (7). Additionally, altera...

The illnesses associated with bacterial superantigens (SAgs) such as food poisoning and toxic shock syndrome, as well as the emerging threat of purpura fulminans and community-associated methicillin-resistant S. aureus producer of SAgs, emphasize the importance of a better characterization of SAg binding to their natural ligands, which would allow the development of drugs or biological reagents able to neutralize their action. SAgs are toxins that bind major histocompatibility complex class II molecules (MHC-II) and T-cell receptors (TCR), in a nonconventional manner, inducing T-cell activation that leads to production of cytokines such as tumor necrosis factor and interleukin-2, which may result in acute toxic shock. Previously, we cloned and expressed a new natural variant of staphylococcal enterotoxin G (SEG) and evaluated its ability to stimulate in vivo murine T-cell subpopulations. We found an early, strong, and widespread stimulation of mouse Vbeta8.2 T-cells when compared with other SAgs member of the SEB subfamily. In search for the reason of the strong mitogenic potency, we determined the SEG crystal structure by X-ray crystallography to 2.2 A resolution and analyzed SEG binding to mVbeta8.2 and MHC-II. Calorimetry and SPR analysis showed that SEG has an affinity for mVbeta8.2 40 to 100-fold higher than that reported for other members of SEB subfamily, and the highest reported for a wild type SAg-TCR couple. We also found that mutations introduced in mVbeta8.2 to produce a high affinity mutant for other members of the SEB subfamily do not greatly affect binding to SEG. Crystallographic analysis and docking into mVbeta8.2 in complex with SEB, SEC3, and SPEA showed that the deletions and substitution of key amino acids remodeled the putative surface of the mVbeta8.2 binding site without affecting the binding to MHC-II. This results in a SAg with improved binding to its natural ligands, which may confer a possible evolutionary advantage for bacterial strains expressing SEG.

TFIID is an essential factor required for RNA polymerase II transcription but remains poorly understood because of its intrinsic complexity. Human TAF5, a 100-kDa subunit of general transcription factor TFIID, is an essential gene and plays a critical role in assembling the 1.2 MDa TFIID complex. We report here a structural analysis of the TAF5 protein. Our structure at 2.2-Å resolution of the TAF5-NTD2 domain reveals an ␣-helical domain with distant structural similarity to RNA polymerase II CTD interacting factors. The TAF5-NTD2 domain contains several conserved clefts likely to be critical for TFIID complex assembly. Our biochemical analysis of the human TAF5 protein demonstrates the ability of the N-terminal half of the TAF5 gene to form a flexible, extended dimer, a key property required for the assembly of the TFIID complex.inititation complex ͉ transcription ͉ x-ray crystallography ͉ protein-protein interaction T he general transcription factor TFIID plays a central role in the recognition of core promoter elements and is used for accurate transcription initiation by RNA Pol II for a large class of genes. Recent studies in yeast indicate that the majority of genes present are TFIID dependent (1). TFIID is a multiprotein complex composed of the TATA box-binding protein (TBP) and 14 other TBP-associated factors (TAFs) which have been highly conserved during eukaryotic evolution (2). TFIID is the only general transcription factor with specific TATA box binding activity and has been shown to initiate recruitment of the other general transcription factors (TFIIA, TFIIB, TFIIE, TFIIF, TFIIH) along with RNA Pol II into a functional preinitiation complex (PIC) that forms at the start site of Pol II genes.TAF subunits seem to serve multiple functions within TFIID holocomplex. For instance, hTAF6 and hTAF9 have been reported to interact with the downstream promoter element, whereas TAF1 and TAF2 have been shown to bind to the initiator. The specific contacts with the promoter DNA by TFIID support basal transcription from promoters containing these elements and have revealed the role of several TAFs on transcription (3-6). TAF1, the largest subunit of TFIID is known to harbor multiple enzymatic activities (7-9) and is also involved in chromatin transactions by using its double bromodomains to contact acetylated histone tails (10, 11). Furthermore, many of the TAFs have been shown to be involved in interactions with gene specific activators and other general transcription factors either to stabilize the preinitiation complex (12) or to induce structural changes in them (13). TAFs are not only restricted to the TFIID complex but also can be found in the yeast SAGA (Spt-Ada-GCN5-acetyltransferase complex), human STAGA (Spt3-TAF9-Ada-GCN5-acetyltransferase complex), PCAF (p300/CBP-associated factor), or TFTC (TBP-free TAFIIcontaining complex) (14-16). The TAFs within these multiprotein complexes are involved in extensive protein-protein interactions and various studies indicate the emerging role of TAFs as cofactors wit...