Structure of SAGA and mechanism of TBP deposition on gene promoters.

SAGA (Spt-Ada-Gcn5-acetyltransferase) is a 19-subunit complex that stimulates transcription via two chromatin-modifying enzymatic modules and by delivering the TATA box binding protein (TBP) to nucleate the pre-initiation complex on DNA, a pivotal event in the expression of protein-encoding genes1. Here we present the structure of yeast SAGA with bound TBP. The core of the complex is resolved at 3.5 Å resolution (0.143 Fourier shell correlation). The structure reveals the intricate network of interactions that coordinate the different functional domains of SAGA and resolves an octamer of histone-fold domains at the core of SAGA. This deformed octamer deviates considerably from the symmetrical analogue in the nucleosome and is precisely tuned to establish a peripheral site for TBP, where steric hindrance represses binding of spurious DNA. Complementary biochemical analysis points to a mechanism for TBP delivery and release from SAGA that requires transcription factor IIA and whose efficiency correlates with the affinity of DNA to TBP. We provide the foundations for understanding the specific delivery of TBP to gene promoters and the multiple roles of SAGA in regulating gene expression.

Optimization of Sample Preparation for the Observation of Macromolecular Complexes by Electron (cryo-)Microscopy.

Electron microscopy is a powerful tool for studying the homogeneity and structure of biomolecular complexes. The small wavelength of electron and the availability of electron optics enable the direct visualization of macromolecular assemblies in a large range of sizes between 5 and 100 nm. This informs us about the degree of multimerization or aggregation and provides precise information about their general shape and dimensions. When combined with sophisticated image analysis protocols, three-dimensional (3D) information can be gained from 2D projections of the sample, leading to a structural description. When intermediate steps of a reaction can be imaged, insights into the mode of action of macromolecules can be gained, and structure-function relations can be established. However, the way the sample is prepared for its observation within the vacuum of an electron microscope determines the information that can be retrieved from the experiment. We will review two commonly used specimen preparation protocols for subsequent single-particle electron microscopy observation, namely negative staining and vitrification.

Atomic structure of the SAGA complex and it's interaction with TBP.

The transcription of eukaryotic protein genes is controlled by a plethora of proteins which act together in multi-component complexes to facilitate the DNA dependent RNA polymerase II (Pol II) enzyme to bind to the transcription start site and to generate messenger RNA from the gene's coding sequence. The protein that guides the transcription machinery to the exact transcription start site is called TATA-box Binding Protein, or TBP. TBP is part of two large protein complexes involved in Pol II transcription, TFIID and SAGA. The two complexes share several subunits implicated in the interaction with TBP and contain proteins with structural elements highly homologous to nucleosomal histones. Despite the intensive study of transcription initiation, the mode of interaction of TBP with these complexes and its release upon DNA binding was elusive. In this study we demonstrate the quasi-atomic model of SAGA in complex with TBP. The structure reveals the intricate network of interactions that coordinate the different functional domains of SAGA and resolves a deformed octamer of histone-fold domains at the core of SAGA. This deformed octamer is precisely tuned to establish a peripheral site for TBP binding, where it is protected by steric hindrance against the binding of spurious DNA. Complementary biochemical analysis points to a mechanism for TBP delivery and release from SAGA that requires the general transcription factor TFIIA and whose efficiency correlates with the affinity of DNA to TBP.As the TBP binding machinery is highly similar in TFIID and SAGA, we demonstrated a universal mechanism of how TBP is delivered to gene promoters during transcription initiation.

La transcription des gènes des protéines eucaryotes est contrôlée par une pléthore de protéines agissant de concert sous forme de complexes multi-composants pour faciliter la liaison de l'enzyme ARN polymérase II ADN-dépendante (Pol II) au site d'initiation de la transcription et pour générer un ARN messager à partir de la séquence codante du gène. La protéine qui guide la machinerie de transcription vers le site d'initiation de la transcription est appelée protéine de liaison à la boîte TATA, ou TBP. TBP fait partie de deux complexes protéiques impliqués dans la transcription par la Pol II, TFIID et SAGA. Les deux complexes partagent plusieurs sous-unités impliquées dans l'interaction avec TBP et comportent des protéines présentant des éléments structuraux hautement homologues aux histones nucléosomiques. Malgré l'étude intensive de l'initiation de la transcription, le mode d'interaction de TBP avec ces complexes ainsi que sa libération lors de sa liaison de l'ADN étaient évasifs. Dans cette étude, nous avons déterminé un modèle quasi-atomique de SAGA en complexe avec TBP. La structure révèle le réseau d'interactions qui coordonnent les différents domaines fonctionnels de SAGA et résout un octamère déformé des domaines homologues aux histones au cœur de SAGA. Cet octamère déformé est précisément adapté pour établir un site périphérique de liaison à TBP, où ce dernier est protégé par une inhibition stérique contre la fixation d'un ADN parasite. L'analyse biochimique complémentaire a mis en évidence un mécanisme de libération de TBP de SAGA qui nécessite le facteur de transcription général TFIIA et dont l'efficacité corrèle avec l'affinité de l'ADN pour TBP.Comme le mécanisme de liaison de TBP est très similaire dans TFIID et SAGA, nous avons mis en évidence un mécanisme universel décrivant la manière dont TBP est délivré aux promoteurs de gènes lors de l'initiation de la transcription.

Molecular architecture of potassium chloride co-transporter KCC2.

KCC2 is a neuron specific K+-Cl- co-transporter that controls neuronal chloride homeostasis, and is critically involved in many neurological diseases including brain trauma, epilepsies, autism and schizophrenia. Despite significant accumulating data on the biology and electrophysiological properties of KCC2, structure-function relationships remain poorly understood. Here we used calixarene detergent to solubilize and purify wild-type non-aggregated and homogenous KCC2. Specific binding of inhibitor compound VU0463271 was demonstrated using surface plasmon resonance (SPR). Mass spectrometry revealed glycosylations and phosphorylations as expected from functional KCC2. We show by electron microscopy (EM) that KCC2 exists as monomers and dimers in solution. Monomers are organized into "head" and "core" domains connected by a flexible "linker". Dimers are asymmetrical and display a bent "S-shape" architecture made of four distinct domains and a flexible dimerization interface. Chemical crosslinking in reducing conditions shows that disulfide bridges are involved in KCC2 dimerization. Moreover, we show that adding a tag to the C-terminus is detrimental to KCC2 function. We postulate that the conserved KCC2 C-ter may be at the interface of dimerization. Taken together, our findings highlight the flexible multi-domain structure of KCC2 with variable anchoring points at the dimerization interface and an important C-ter extremity providing the first in-depth functional architecture of KCC2.