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Centromeric chromatin is a subset of chromatin structure and governs chromosome segregation. The centromere is composed of both CENP-A nucleosomes (CENP-A(nuc)) and H3 nucleosomes (H3(nuc)) and is enriched with alpha-satellite (alpha-sat) DNA repeats. These CENP-A(nuc) have a different structure than H3(nuc), decreasing the base pairs (bp) of wrapped DNA from 147 bp for H3(nuc) to 121 bp for CENP-A(nuc). All these factors can contribute to centromere function. We investigated the interaction of H3(nuc) and CENP-A(nuc) with NF-kappaB, a crucial transcription factor in regulating immune response and inflammation. We utilized atomic force microscopy (AFM) to characterize complexes of both types of nucleosomes with NF-kappaB. We found that NF-kappaB unravels H3(nuc), removing more than 20 bp of DNA, and that NF-kappaB binds to the nucleosomal core. Similar results were obtained for the truncated variant of NF-kappaB comprised only of the Rel homology domain and missing the transcription activation domain (TAD), suggesting that RelA(TAD) is not critical in unraveling H3(nuc). By contrast, NF-kappaB did not bind to or unravel CENP-A(nuc). These findings with different affinities for two types of nucleosomes to NF-kappaB may have implications for understanding the mechanisms of gene expression in bulk and centromere chromatin. 

Abstract Binding and unbinding of transcription factors to DNA are kinetically controlled to regulate the transcriptional outcome. Control of the release of the transcription factor NF-κB from DNA is achieved through accelerated dissociation by the inhibitor protein IκBα. Using single-molecule FRET, we observed a continuum of conformations of NF-κB in free and DNA-bound states interconverting on the subseconds to minutes timescale, comparable to in vivo binding on the seconds timescale, suggesting that structural dynamics directly control binding kinetics. Much of the DNA-bound NF-κB is partially bound, allowing IκBα invasion to facilitate DNA dissociation. IκBα induces a locked conformation where the DNA-binding domains of NF-κB are too far apart to bind DNA, whereas a loss-of-function IκBα mutant retains the NF-κB conformational ensemble. Overall, our results suggest a novel mechanism with a continuum of binding modes for controlling association and dissociation of transcription factors. 

Abstract The Cullin 5 (CUL5) Ring E3 ligase uses adaptors Elongins B and C (ELOB/C) to bind different SOCS-box-containing substrate receptors, determining the substrate specificity of the ligase. The 18-member ankyrin and SOCS box (ASB) family is the largest substrate receptor family. Here we report cryo-EM data for the substrate, creatine kinase (CKB) bound to ASB9-ELOB/C, and for full-length CUL5 bound to the RING protein, RBX2, which binds various E2s. To date, no full structures are available either for a substrate-bound ASB nor for CUL5. Hydrogen–deuterium exchange (HDX-MS) mapped onto a full structural model of the ligase revealed long-range allostery extending from the substrate through CUL5. We propose a revised allosteric mechanism for how CUL-E3 ligases function. ASB9 and CUL5 behave as rigid rods, connected through a hinge provided by ELOB/C transmitting long-range allosteric crosstalk from the substrate through CUL5 to the RBX2 flexible linker. 

Abstract To function, biomolecules require sufficient specificity of interaction as well as stability to live in the cell while still being able to move. Thermodynamic stability of only a limited number of specific structures is important so as to prevent promiscuous interactions. The individual interactions in proteins, therefore, have evolved collectively to give funneled minimally frustrated landscapes but some strategic parts of biomolecular sequences located at specific sites in the structure have been selected to be frustrated in order to allow both motion and interaction with partners. We describe a framework efficiently to quantify and localize biomolecular frustration at atomic resolution by examining the statistics of the energy changes that occur when the local environment of a site is changed. The location of patches of highly frustrated interactions correlates with key biological locations needed for physiological function. At atomic resolution, it becomes possible to extend frustration analysis to protein-ligand complexes. At this resolution one sees that drug specificity is correlated with there being a minimally frustrated binding pocket leading to a funneled binding landscape. Atomistic frustration analysis provides a route for screening for more specific compounds for drug discovery.