Comparison Between the Structure-Function Relationship in the Wild Type Gαi1 Protein and Its Oncogenic Mutant

Many signal transduction pathways are regulated by guanine nucleotide-binding (G?) proteins, which function as molecular switches fluctuating between active and inactive conformations. Proper function depends on three flexible switch regions that are involved in the relatively slow hydrolysis of GTP. Deep sequencing studies have found mutations in the GNAS and GNAI1 genes involved in tumorigenesis, among which include a mutation corresponding to a highly conserved arginine residue in the switch II region. A mutation in GNAI1 encoding an R208Q change in G?i1 has been linked to intestinal cancers. We investigated the molecular basis of oncogenesis of this mutant by studying the kinetics of nucleotide binding and single-turnover GTP hydrolysis. We demonstrated that, relative to the corresponding wild-type proteins, this mutation hindered nucleotide exchange; however, the rate of GTP hydrolysis was lower in R208Q G?i1. The R208Q G?i1 mutant was crystallized and its X-ray structure was compared to that of the wild-type protein and was used to conduct molecular dynamics simulations. Furthermore, we investigate the folding and structural integrity of the protein with three spectroscopic techniques. We show that both the active and inactive conformations of have similar melting temperatures, which are comparable to the inactive conformation of the wild-type protein and lower relative to the active conformation. These studies suggested that changes in the rates of hydrolysis can be attributed to alterations in the microenvironments of the nucleotide binding site which seemingly destabilize the switch II region but do not perturb the surface of the protein. The mutation presumably results in a decrease in the production of the secondary messenger cAMP via its interaction with the effector adenylyl cyclase that might promote oncogenesis in tumor cells

Observational Tests of Damping by Resonant Absorption in Coronal Loop Oscillations

One of the proposed damping mechanisms of coronal (transverse) loop oscillations in the kink-mode is resonant absorption as a result of the Alfven speed variation at the outer boundary of coronal loops. Analytical expressions for the period and damping time exist for loop models with thin non-uniform boundaries. Here we measure the thickness of the non-uniform layer in oscillating loops for 11 events, by forward-fitting of the cross-sectional density profile and line-of-sight integration to the cross-sectional fluxes observed with TRACE 171 A. This way we model the internal and external electron density of the coronal plasma in oscillating loops. This allows us to test the theoretically predicted damping rates for thin boundaries as function of the density ratio. We find that the density ratio predicted by the damping time is higher than the density ratio estimated from the background fluxes. The lower densities modeled from the background fluxes are likely to be a consequence of the neglected hotter plasma that is not detected with the TRACE 171 A filter. Taking these correction into account, resonant absorption predicts damping times of kink-mode oscillations that are commensurable with the observed ones and provides a new diagnostic of the density contrast of oscillating loops.Comment: 10 Figure

Mediator and cohesin connect gene expression and chromatin architecture

Transcription factors control cell-specific gene expression programs through interactions with diverse coactivators and the transcription apparatus. Gene activation may involve DNA loop formation between enhancer-bound transcription factors and the transcription apparatus at the core promoter, but this process is not well understood. Here we report that mediator and cohesin physically and functionally connect the enhancers and core promoters of active genes in murine embryonic stem cells. Mediator, a transcriptional coactivator, forms a complex with cohesin, which can form rings that connect two DNA segments. The cohesin-loading factor Nipbl is associated with mediator–cohesin complexes, providing a means to load cohesin at promoters. DNA looping is observed between the enhancers and promoters occupied by mediator and cohesin. Mediator and cohesin co-occupy different promoters in different cells, thus generating cell-type-specific DNA loops linked to the gene expression program of each cell.National Institutes of Health (U.S.) (Fellowship)Canadian Institutes of Health Research (Research Fellowship)National Institutes of Health (U.S.) (Grant R01 HG002668

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements