The Mollow triplets under few-photon excitation

Resonant excitation is an essential tool in the development of semiconductor quantum dots (QDs) for quantum information processing. One central challenge is to enable a transparent access to the QD signal without post-selection information loss. A viable path is through cavity enhancement, which has successfully lifted the resonantly scattered field strength over the laser background under \emph{weak} excitation. Here, we extend this success to the \emph{saturation} regime using a QD-micropillar device with a Purcell factor of 10.9 and an ultra-low background cavity reflectivity of just 0.0089. We achieve a signal to background ratio of 50 and an overall system responsivity of 3~\%, i.e., we detect on average 0.03 resonantly scattered single photons for every incident laser photon. Raising the excitation to the few-photon level, the QD response is brought into saturation where we observe the Mollow triplets as well as the associated cascade single photon emissions, without resort to any laser background rejection technique. Our work offers a new perspective toward QD cavity interface that is not restricted by the laser background.Comment: 8 Figures and 9 Pages. Comments are welcom

Crystal structure of the zinc-bound HhoA protease from Synechocystis sp PCC 6803

The high temperature requirement A (HtrA) proteases are oligomeric serine proteases essential for protein quality control. HtrA homolog A (HhoA) from the photosynthetic cyanobacterium Synechocystis sp. PCC 6803 assembles into a proteolytically active hexamer. Herein, we present the crystal structure of the hexameric HhoA in complex with the copurified peptide. Our data indicate the presence of three methionines in close proximity to the peptide-binding site of the PDZ domain. Unexpectedly, we observed that a zinc ion is accommodated within the central channel formed by a HhoA trimer. However, neither calcium nor magnesium showed affinity for HhoA. The role of the zinc ion in HhoA was tested in an in vitro proteolytic assay against the nonspecific substrate -casein and was found to be inhibitory. Our findings provide insights into the regulation of HhoA by a redox-related mechanism involving methionine residues and by zinc ion-binding within the central channel

Structural basis for plant lutein biosynthesis from alpha-carotene

Two cytochrome P450 enzymes, CYP97A3 and CYP97C1, catalyze hydroxylations of the beta- and epsilon-rings of alpha-carotene to produce lu- tein. Chirality is introduced at the C-3 atom of both rings, and the reactions are both pro-3 R-stereospecific. We determined the crys- tal structures of CYP97A3 in substrate -free and complex forms with a nonnatural substrate and the structure of CYP97C1 in a detergent -bound form. The structures of CYP97A3 in different states show the substrate channel and the structure of CYP97C1 bound with octylthioglucoside confirms the binding site for the carotenoid substrate. Biochemical assays confirm that the ferredoxin- NADP + reductase (FNR) -ferredoxin pair is used as the redox part- ner. Details of the pro -3 R stereospecificity are revealed in the retinal -bound CYP97A3 structure. Further analysis indicates that the CYP97B clan bears similarity to the beta-ring -specific CYP97A clan. Overall, our research describes the molecular basis for the last steps of lutein biosynthesis

Structure of the Arabidopsis thaliana NADPH-cytochrome P450 reductase 2 (ATR2) provides insight into its function

Members of the cytochrome P450 family catalyze a variety of mono-oxygenase reactions, and for the eukaryotic membrane-bound members, NADPH is typically used as the reducing agent. The flavoprotein NADPH-cytochrome P450 reductase (CPR) enables electron transfer from NADPH to cytochrome P450 via its flavin cofactors. ATR2 is one of the two authentic CPR genes in the genome of the model plant Arabidopsis thaliana, and its product has been physiologically and kinetically characterized. Here, we report the 2.3 angstrom structure of Arabidopsis thaliana NADPH-cytochrome P450 reductase 2 (ATR2) and find that the position of the two flavin cofactors differs from that of other known CPR structures. Mutation of residues related to possible interflavin electron transfer retains the reductase activity of ATR2, which suggests a direct electron transfer pathway between the flavins. In contrast, mutation of a single residue (R708) mediating interdomain interaction abolishes this activity. Because this residue is only conserved in plant CPRs, we speculate a plant-specific working mechanism as observed in ATR2

Structural basis for copper/silver binding by the Synechocystis metallochaperone CopM

Copper homeostasis integrates multiple processes from sensing to storage and efflux out of the cell. CopM is a cyanobacterial metallochaperone, the gene for which is located upstream of a two-component system for copper resistance, but the molecular basis for copper recognition by this four-helical bundle protein is unknown. Here, crystal structures of CopM in apo, copper-bound and silver-bound forms are reported. Monovalent copper/silver ions are buried within the bundle core; divalent copper ions are found on the surface of the bundle. The monovalent copper/silver-binding site is constituted by two consecutive histidines and is conserved in a previously functionally unknown protein family. The structural analyses show two conformational states and suggest that flexibility in the first alpha-helix is related to the metallochaperone function. These results also reveal functional diversity from a protein family with a simple fourhelical fold

Structural basis for copper/silver binding by the Synechocystis metallochaperone CopM

Copper homeostasis integrates multiple processes from sensing to storage and efflux out of the cell. CopM is a cyanobacterial metallochaperone, the gene for which is located upstream of a two-component system for copper resistance, but the molecular basis for copper recognition by this four-helical bundle protein is unknown. Here, crystal structures of CopM in apo, copper-bound and silver-bound forms are reported. Monovalent copper/silver ions are buried within the bundle core; divalent copper ions are found on the surface of the bundle. The monovalent copper/silver-binding site is constituted by two consecutive histidines and is conserved in a previously functionally unknown protein family. The structural analyses show two conformational states and suggest that flexibility in the first alpha-helix is related to the metallochaperone function. These results also reveal functional diversity from a protein family with a simple fourhelical fold

Activation of GSDME by Lithospermum erythrorhizon drives pyroptotic cell death

Objective: To reveal GSDME-executed pyroptosis in cancer cells induced by the Chinese traditional herbal medicine plant Lithospermum erythrorhizon (L. erythrorhizon, Zi Cao) and to investigate the potential mechanism. Methods: L. erythrorhizon was extracted by ultrasonication in 95% ethanol, and determined using high-performance liquid chromatography (HPLC). HeLa, A549, SW620, HEK-293 T, THP-1, K562, Raw264.7 and MDA-MB-231 cell lines were used to investigate the morphology and mechanism of pyroptosis induced by L. erythrorhizon. The lactate dehydrogenase (LDH) release, propidium iodide (PI)/Hoechst double-staining, and pyroptosis reconstitution experiments were performed to study L. erythrorhizon -induced cell pyroptosis. Results: Compared with the death inhibitor, PI/Hoechst and LDH release experiments, we found that L. erythrorhizon induced pyroptosis. Recombination and western blot experiments confired that L. erythrorhizon induced GSDME cleavage, which drives pyroptosis. This phenomenon is conserved in several cancer cell lines that might be triggered by caspase family proteases. The mechanism of L. erythrorhizon inducing pyroptosis is widely found in tumor cells. Conclusion: Our findings not only explain how L. erythrorhizon triggers cancer cell pyroptosis, but also provide mechanistic insights to guide its clinical application in the future