No abstract

The tumour suppressor p53 is the most frequently mutated gene in human cancer. Reactivation of mutant p53 by small molecules is an exciting potential cancer therapy. Although several compounds restore wild-type function to mutant p53, their binding sites and mechanisms of action are elusive. Here computational methods identify a transiently open binding pocket between loop L1 and sheet S3 of the p53 core domain. Mutation of residue Cys124, located at the centre of the pocket, abolishes p53 reactivation of mutant R175H by PRIMA-1, a known reactivation compound. Ensemble-based virtual screening against this newly revealed pocket selects stictic acid as a potential p53 reactivation compound. In human osteosarcoma cells, stictic acid exhibits dose-dependent reactivation of p21 expression for mutant R175H more strongly than does PRIMA-1. These results indicate the L1/S3 pocket as a target for pharmaceutical reactivation of p53 mutants.

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The bromodomain-containing proteins BRD9 and BRD7 are part of the human SWI/SNF chromatin-remodeling complexes BAF and PBAF. To date, no selective inhibitor for BRD7/9 has been reported despite its potential value as a biological tool or as a lead for future therapeutics. The quinolone-fused lactam LP99 is now reported as the first potent and selective inhibitor of the BRD7 and BRD9 bromodomains. Development of LP99 from a fragment hit was expedited through balancing structure-based inhibitor design and biophysical characterization against tractable chemical synthesis: Complexity-building nitro-Mannich/lactamization cascade processes allowed for early structure–activity relationship studies whereas an enantioselective organocatalytic nitro-Mannich reaction enabled the synthesis of the lead scaffold in enantioenriched form and on scale. This epigenetic probe was shown to inhibit the association of BRD7 and BRD9 to acetylated histones in vitro and in cells. Moreover, LP99 was used to demonstrate that BRD7/9 plays a role in regulating pro-inflammatory cytokine secretion.

Three plant xanthophylls are components of the xanthophyll cycle in which, upon exposure of leaves to high light, the enzyme violaxanthin de-epoxidase (VDE) transforms violaxanthin into zeaxanthin via the intermediate antheraxanthin. Previous work (1) showed that xanthophylls are bound to Lhc proteins and that substitution of violaxanthin with zeaxanthin induces conformational changes and fluorescence quenching by thermal dissipation. We have analyzed the efficiency of different Lhc proteins to exchange violaxanthin with zeaxanthin both in vivo and in vitro. Light stress of Zea mays leaves activates VDE, and the newly formed zeaxanthin is found primarily in CP26 and CP24, whereas other Lhc proteins show a lower exchange capacity. The de-epoxidation system has been reconstituted in vitro by using recombinant Lhc proteins, recombinant VDE, and monogalactosyl diacylglycerol (MGDG) to determine the intrinsic capacity for violaxanthin-to-zeaxanthin exchange of individual Lhc gene products. Again, CP26 was the most efficient in xanthophyll exchange. Biochemical and spectroscopic analysis of individual Lhc proteins after de-epoxidation in vitro showed that xanthophyll exchange occurs at the L2-binding site. Xanthophyll exchange depends on low pH, implying that access to the binding site is controlled by a conformational change via lumenal pH. These findings suggest that the xanthophyll cycle participates in a signal transduction system acting in the modulation of light harvesting versus thermal dissipation in the antenna system of higher plants.