Bisphenol A (BPA) is an industrial compound and a well known endocrine-disrupting chemical with estrogenic activity. The widespread exposure of individuals to BPA is suspected to affect a variety of physiological functions, including reproduction, development, and metabolism. Here we report that the mechanisms by which BPA and two congeners, bisphenol AF and bisphenol C (BPC), bind to and activate estrogen receptors (ER) α and β differ from that used by 17β-estradiol. We show that bisphenols act as partial agonists of ERs by activating the N-terminal activation function 1 regardless of their effect on the C-terminal activation function 2, which ranges from weak agonism (with BPA) to antagonism (with BPC). Crystallographic analysis of the interaction between bisphenols and ERs reveals two discrete binding modes, reflecting the different activities of compounds on ERs. BPA and 17β-estradiol bind to ERs in a similar fashion, whereas, with a phenol ring pointing toward the activation helix H12, the orientation of BPC accounts for the marked antagonist character of this compound. Based on structural data, we developed a protocol for in silico evaluation of the interaction between bisphenols and ERs or other members of the nuclear hormone receptor family, such as estrogen-related receptor γ and androgen receptor, which are two known main targets of bisphenols. Overall, this study provides a wealth of tools and information that could be used for the development of BPA substitutes devoid of nuclear hormone receptor-mediated activity and more generally for environmental risk assessment.

Chromatin insulators are genetic elements implicated in the organization of chromatin and the regulation of transcription. In Drosophila, different insulator types were characterized by their locus-specific composition of insulator proteins and co-factors. Insulators mediate specific long-range DNA contacts required for the three dimensional organization of the interphase nucleus and for transcription regulation, but the mechanisms underlying the formation of these contacts is currently unknown. Here, we investigate the molecular associations between different components of insulator complexes (BEAF32, CP190 and Chromator) by biochemical and biophysical means, and develop a novel single-molecule assay to determine what factors are necessary and essential for the formation of long-range DNA interactions. We show that BEAF32 is able to bind DNA specifically and with high affinity, but not to bridge long-range interactions (LRI). In contrast, we show that CP190 and Chromator are able to mediate LRI between specifically-bound BEAF32 nucleoprotein complexes in vitro. This ability of CP190 and Chromator to establish LRI requires specific contacts between BEAF32 and their C-terminal domains, and dimerization through their N-terminal domains. In particular, the BTB/POZ domains of CP190 form a strict homodimer, and its C-terminal domain interacts with several insulator binding proteins. We propose a general model for insulator function in which BEAF32/dCTCF/Su(HW) provide DNA specificity (first layer proteins) whereas CP190/Chromator are responsible for the physical interactions required for long-range contacts (second layer). This network of organized, multi-layer interactions could explain the different activities of insulators as chromatin barriers, enhancer blockers, and transcriptional regulators, and suggest a general mechanism for how insulators may shape the organization of higher-order chromatin during cell division. Chromatin insulators mediate specific long-range DNA interactions required for the three dimensional organization of the interphase nucleus and for transcription regulation, but the mechanisms underlying the formation of these interactions is currently unknown. In this manuscript, we investigate the molecular associations between different protein components of insulators (BEAF32, CP190 and Chromator) by biochemical and biophysical means, and develop a novel biophysical assay to determine what factors are necessary and essential for the formation of long-range DNA interactions (LRI). Importantly, we show that CP190 and Chromator are able to mediate LRIs between specifically-bound BEAF32 nucleoprotein complexes. This ability of CP190 and Chromator to establish LRI requires specific contacts between BEAF32 and their C-terminal domains, and dimerization through their N-terminal domains. In particular, the BTB/POZ domains of CP190 form a strict homodimer. We propose a general model for insulator function in which BEAF32/dCTCF/Su(HW) provide DNA specificity, whereas CP190/Chromator are responsible for the physical interactions required for long-range contacts. This network of organized, multi-layer interactions could explain the different activities of insulators, and suggest a general mechanism for how insulators may shape the organization of higher-order chromatin during cell division.

Toxoplasma gondii is an obligate intracellular parasite for which the discharge of apical organelles named rhoptries is a key event in host cell invasion. Among rhoptry proteins, ROP2, which is the prototype of a large protein family, is translocated in the parasitophorous vacuole membrane during invasion. The ROP2 family members are related to protein-kinases, but only some of them are predicted to be catalytically active, and none of the latter has been characterized so far. We show here that ROP18, a member of the ROP2 family, is located in the rhoptries and re-localises at the parasitophorous vacuole membrane during invasion. We demonstrate that a recombinant ROP18 catalytic domain (amino acids 243–539) possesses a protein-kinase activity and phosphorylate parasitic substrates, especially a 70-kDa protein of tachyzoites. Furthermore, we show that overexpression of ROP18 in transgenic parasites causes a dramatic increase in intra-vacuolar parasite multiplication rate, which is correlated with kinase activity. Therefore, we demonstrate, to our knowledge for the first time, that rhoptries can discharge active protein-kinases upon host cell invasion, which can exert a long-lasting effect on intracellular parasite development and virulence. Apicomplexa are unicellular eukaryotes that cause a number of diseases, including malaria. Most of them are obligate intracellular parasites, developing in a parasitophorous vacuole (PV) within their host cell. PV formation during invasion is associated with the exocytosis of parasite secretory organelles named rhoptries, whose role is unknown. Toxoplasma gondii is a model Apicomplexa responsible for toxoplasmosis, a fatal congenital or opportunistic infection in humans and animals. We have studied a novel rhoptry protein dubbed ROP18, which is translocated to the PV membrane upon invasion. ROP18 belongs to a family of rhoptry proteins that share homologies with serine-threonine kinases, but those described so far lack residues critical for enzyme activity. We show that ROP18 possesses all the features needed to be active, and we experimentally demonstrate this activity, which phosphorylates at least one parasite protein. We show that overexpression of ROP18 causes a dramatic increase in parasite multiplication rate that is correlated with kinase activity, and likely dependent on a PV membrane modification. We therefore demonstrate that rhoptries can discharge active protein-kinases upon invasion, which can exert a long-lasting effect on intracellular parasite development and virulence.