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The human calcium-sensing receptor (CaSR) is a class C G protein-coupled receptor (GPCR) responsible for maintaining Ca 2+ homeostasis in the blood. The general consensus is that extracellular Ca 2+ is the principal agonist of CaSR. Aliphatic and aromatic L-amino acids, such as L-Phe and L-Trp, increase the sensitivity of CaSR towards Ca 2+ and are considered allosteric activators. Crystal structures of the extracellular domain (ECD) of CaSR dimer have demonstrated Ca 2+ and L-Trp binding sites and conformational changes of the ECD upon Ca 2+ /L-Trp binding. However, it remains to be understood at the structural level how Ca 2+ /L-Trp binding to the ECD leads to conformational changes in transmembrane domains (TMDs) and consequent CaSR activation. Here, we determined the structures of full-length human CaSR in the inactive state, Ca 2+ - or L-Trp-bound states, and Ca 2+ /L-Trp-bound active state using single-particle cryo-electron microscopy. Structural studies demonstrate that L-Trp binding induces the closure of the Venus flytrap (VFT) domain of CaSR, bringing the receptor into an intermediate active state. Ca 2+ binding relays the conformational changes from the VFT domains to the TMDs, consequently inducing close contact between the two TMDs of dimeric CaSR, activating the receptor. Importantly, our structural and functional studies reveal that Ca 2+ ions and L-Trp activate CaSR cooperatively. Amino acids are not able to activate CaSR alone, but can promote the receptor activation in the presence of Ca 2+ . Our data provide complementary insights into the activation of class C GPCRs and may aid in the development of novel drugs targeting CaSR.

Respiratory complexes are massive, membrane-embedded scaffolds that position redox cofactors so as to permit electron transfer coupled to the movement of protons across a membrane. Gong et al. used cryo–electron microscopy to determine a structure of a stable assembly of mycobacterial complex III–IV, in which a complex III dimer is sandwiched between two complex IV monomers. A potential direct electron transfer path stretches from the quinone oxidizing centers in complex III to the oxygen reduction centers in complex IV. A loosely associated superoxide dismutase may play a role in detoxifying superoxide produced from uncoupled oxygen reduction. Science , this issue p. eaat8923 A mycobacterial respiratory supercomplex forgoes soluble electron carriers and associates with superoxide dismutase. We report a 3.5-angstrom-resolution cryo–electron microscopy structure of a respiratory supercomplex isolated from Mycobacterium smegmatis. It comprises a complex III dimer flanked on either side by individual complex IV subunits. Complex III and IV associate so that electrons can be transferred from quinol in complex III to the oxygen reduction center in complex IV by way of a bridging cytochrome subunit. We observed a superoxide dismutase-like subunit at the periplasmic face, which may be responsible for detoxification of superoxide formed by complex III. The structure reveals features of an established drug target and provides a foundation for the development of treatments for human tuberculosis.

The central dogma of modern biology underscores the pivotal roles proteins play in diverse biological processes, the study of which necessitates advanced methods to produce proteins with precision and versatility. Chemical protein synthesis, a powerful approach utilizing chemical reactions for the de novo construction of structurally accurate proteins, has emerged as a transformative tool for studying proteins and generating protein derivatives/mimics inaccessible by natural biological machinery, including post-translationally modified proteins, proteins comprised of unnatural amino acids, as well as mirror-image proteins. This review summarizes recent strides in synthetic method developments for chemical protein synthesis, including innovative techniques in solid-phase peptide synthesis, the challenges presented by difficult sequences in either synthesis or folding and the exploration of novel ligation reactions using both chemical and enzymatic methods. Furthermore, the review also delves into newly developed protocols for site-selective protein modifications and the generation of stapled or macrocyclized peptides/mini-proteins, highlighting the power of chemical methods to make structurally diverse proteins. Recent applications of synthetic proteins in investigating post-translational modifications (phosphorylation, lipidation, glycosylation, ubiquitination, etc. ), mirror-image biological processes and drug development are further discussed. Together, these topics provide a comprehensive overview of the current landscape of chemical protein synthesis.

Repression of gene expression by protein complexes of the Polycomb group is a fundamental mechanism that governs embryonic development and cell-type specification 1 – 3 . The Polycomb repressive deubiquitinase (PR-DUB) complex removes the ubiquitin moiety from monoubiquitinated histone H2A K119 (H2AK119ub1) on the nucleosome 4 , counteracting the ubiquitin E3 ligase activity of Polycomb repressive complex 1 (PRC1) 5 to facilitate the correct silencing of genes by Polycomb proteins and safeguard active genes from inadvertent silencing by PRC1 (refs. 6 – 9 ). The intricate biological function of PR-DUB requires accurate targeting of H2AK119ub1, but PR-DUB can deubiquitinate monoubiquitinated free histones and peptide substrates indiscriminately; the basis for its exquisite nucleosome-dependent substrate specificity therefore remains unclear. Here we report the cryo-electron microscopy structure of human PR-DUB, composed of BAP1 and ASXL1, in complex with the chromatosome. We find that ASXL1 directs the binding of the positively charged C-terminal extension of BAP1 to nucleosomal DNA and histones H3–H4 near the dyad, an addition to its role in forming the ubiquitin-binding cleft. Furthermore, a conserved loop segment of the catalytic domain of BAP1 is situated near the H2A–H2B acidic patch. This distinct nucleosome-binding mode displaces the C-terminal tail of H2A from the nucleosome surface, and endows PR-DUB with the specificity for H2AK119ub1. The cryo-electron microscopy structure of the Polycomb repressive deubiquitinase (PR-DUB) in complex with the H2AK119ub1 nucleosome provides insight into how the substrate specificity of PR-DUB is achieved.

The mixed lineage leukaemia (MLL) family of proteins (including MLL1–MLL4, SET1A and SET1B) specifically methylate histone 3 Lys4, and have pivotal roles in the transcriptional regulation of genes involved in haematopoiesis and development. The methyltransferase activity of MLL1, by itself severely compromised, is stimulated by the three conserved factors WDR5, RBBP5 and ASH2L, which are shared by all MLL family complexes. However, the molecular mechanism of how these factors regulate the activity of MLL proteins still remains poorly understood. Here we show that a minimized human RBBP5–ASH2L heterodimer is the structural unit that interacts with and activates all MLL family histone methyltransferases. Our structural, biochemical and computational analyses reveal a two-step activation mechanism of MLL family proteins. These findings provide unprecedented insights into the common theme and functional plasticity in complex assembly and activity regulation of MLL family methyltransferases, and also suggest a universal regulation mechanism for most histone methyltransferases. Crystal structures of the SET domains of MLL3 and a mutant MLL1 either unbound or complexed with domains from RBBP5 and ASH2L are determined; a combination of structural, biochemical and computational analyses reveals a two-step activation mechanism of MLL family proteins, which may be relevant for other histone methyltransferases. Activation mechanism for MLL enzymes The SET domain-containing MLL family proteins methylate histone 3 on lysine 4 (H3K4) and have key roles in transcriptional regulation. MLL proteins are catalytically inactive on their own, and have full activity only when bound in a complex with three factors: WDR5, RBBP5, and ASH2L. Yong Chen and colleagues determine crystal structures of the SET domains of MLL3 and a mutant MLL1 in unbound forms or complexed with domains from RBBP5 and ASH2L and the histone H3 substrate. Their results suggest that WDR5 is not directly involved in enzymatic stimulation, and a combination of structural, biochemical and computational analyses reveals a two-step activation mechanism which may be relevant for all histone methyltransferases.

Diheme-containing succinate:menaquinone oxidoreductases (Sdh) are widespread in Gram-positive bacteria but little is known about the catalytic mechanisms they employ for succinate oxidation by menaquinone. Here, we present the 2.8 Å cryo-electron microscopy structure of a Mycobacterium smegmatis Sdh, which forms a trimer. We identified the membrane-anchored SdhF as a subunit of the complex. The 3 kDa SdhF forms a single transmembrane helix and this helix plays a role in blocking the canonically proximal quinone-binding site. We also identified two distal quinone-binding sites with bound quinones. One distal binding site is formed by neighboring subunits of the complex. Our structure further reveals the electron/proton transfer pathway for succinate oxidation by menaquinone. Moreover, this study provides further structural insights into the physiological significance of a trimeric respiratory complex II. The structure of the menaquinone binding site could provide a framework for the development of Sdh-selective anti-mycobacterial drugs. Diheme-containing succinate:menaquinone oxidoreductases (Sdh) are members of the complex II superfamily. Here, the authors present the 2.8 Å cryo-EM structure of Mycobacterium smegmatis Sdh2, which reveals membrane-anchored SdhF as a component of the complex and they discuss the electron/proton transfer pathway in the Sdh2 trimer.

NaK and other non-selective channels are able to conduct both sodium (Na + ) and potassium (K + ) with equally high efficiency. In contrast to previous crystallographic results, we show that the selectivity filter (SF) of NaK in native-like lipid membranes adopts two distinct conformations that are stabilized by either Na + or K + ions. The atomic differences of these conformations are resolved by solid-state NMR (ssNMR) spectroscopy and molecular dynamics (MD) simulations. Besides the canonical K + permeation pathway, we identify a side entry ion-conduction pathway for Na + permeation unique to NaK. Moreover, under otherwise identical conditions ssNMR spectra of the K + selective NaK mutant (NaK2K) reveal only a single conformational state. Therefore, we propose that structural plasticity within the SF and the selection of these conformations by different ions are key molecular determinants for highly efficient conduction of different ions in non-selective cation channels. NaK is a non-selective cation channel that conducts sodium (Na + ) and potassium (K + ) equally well. Here authors use ssNMR and MD simulations to show that the selectivity filter of NaK adopts two conformations in the absence of ions, one of which is preferred by Na + and the other by K + .

Unsaturated fatty acids (UFAs) are essential for functional membrane phospholipids in most bacteria. The bifunctional dehydrogenase/isomerase FabX is an essential UFA biosynthesis enzyme in the widespread human pathogen Helicobacter pylori , a bacterium etiologically related to 95% of gastric cancers. Here, we present the crystal structures of FabX alone and in complexes with an octanoyl-acyl carrier protein (ACP) substrate or with holo-ACP. FabX belongs to the nitronate monooxygenase (NMO) flavoprotein family but contains an atypical [4Fe-4S] cluster absent in all other family members characterized to date. FabX binds ACP via its positively charged α7 helix that interacts with the negatively charged α2 and α3 helices of ACP. We demonstrate that the [4Fe-4S] cluster potentiates FMN oxidation during dehydrogenase catalysis, generating superoxide from an oxygen molecule that is locked in an oxyanion hole between the FMN and the active site residue His182. Both the [4Fe-4S] and FMN cofactors are essential for UFA synthesis, and the superoxide is subsequently excreted by H. pylori as a major resource of peroxide which may contribute to its pathogenic function in the corrosion of gastric mucosa. Helicobacter pylori FabX, a dehydrogenase/isomerase flavoprotein, is required for unsaturated fatty acid synthesis. Here, the authors characterize FabX substrate recognition and catalytic mechanism, and reveal that it contains an atypical [4Fe-4S] cluster, which is essential and participates in the catalytic cycle.

Our skin safeguards the body homeostasis for health and also provides psychological consolation in social life. Natural essential oils are widely used for skin maintenance, while the molecular target and mechanism of action remain largely unknown. Here, we report that citronellal, a plant-derived acyclic monoterpene commonly used for personal care, stimulates skin renewal by promoting keratinocyte proliferation through the activation of TRPV3. We further present cryo-EM structures of human TRPV3 in complex with acyclic monoterpenes, including citronellal, citral, linalool and isodihydrolavandulal, determined at resolutions of 3.1-3.6 Å. Our structural and functional analysis unmasks consistent yet subtly different binding modes within the TRPV3 vanilloid site. Our results elucidate that essential oil ligands activate TRPV3 channels by competitively displacing endogenous lipids from the vanilloid site. Together, these findings identify TRPV3 as the molecular target of natural acyclic monoterpenes for skin renewal, and delineate the structural basis of action, thus being instrumental for moving forward skin healthcare. In this work, the authors report that citronellal stimulates skin renewal by promoting keratinocyte proliferation through the activation of TRPV3. Structural and functional analysis elucidate that essential oil ligands activate TRPV3 channels by competitively displacing endogenous lipids from the vanilloid site. These findings can help move skin healthcare forward.